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# 5.4: Applications of Systems of Equations

Difficulty Level: Advanced Created by: CK-12
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Can you determine two numbers such that the sum of the numbers is 763 and the difference between the same two numbers is 179? How can a system of equations help you?

### Applications of Systems of Equations

A system of linear equations can be used to represent real-world problems. A system of equations is used when there are two variables and you are given two pieces of information about how those variables are related. The examples below will show you some common problems that can be solved using a system of equations.

#### Find the dimensions of the rectangle

The length of a rectangular plot of land is 255 yards longer than the width. If the perimeter is 1206 yards, find the dimensions of the rectangle.

The perimeter of a rectangle is found by the formula P=2l+2w\begin{align*}P=2l+2w\end{align*} where P\begin{align*}P\end{align*} is the perimeter, l\begin{align*}l\end{align*} is the length and w\begin{align*}w\end{align*} is the width. The quantities are the length and the width of the rectangle.

• Let the length of the rectangle be represented by ‘\begin{align*}l\end{align*}’.
• Let the width of the rectangle be represented by ‘\begin{align*}w\end{align*}’.

The equations would be:

• The length is 255 yards longer than the width \begin{align*}\rightarrow \boxed{l=w+255}\end{align*}
• The perimeter is 1206 yards \begin{align*}\rightarrow \boxed{2l+2w=1206}\end{align*}

The system of equations can now be solved to determine the dimensions of the rectangle. In the first equation the length is expressed in terms of the width. Substitution will be used to solve the system of equations.

\begin{align*}& \begin{Bmatrix} l=w+255\\ 2l+2w=1206 \end{Bmatrix}\\ & 2l+2w = 1206 && \text{Substitute} \ (w + 255) \ \text{for} \ l \ \text{in the equation.}\\ & 2({\color{red}w+255})+2w = 1206 && \text{Apply the distributive property.}\\ & {\color{red}2w+510}+2w = 1206 && \text{Simplify}\\ & {\color{red}4w}+510 =1206 && \text{Solve the equation.}\\ & 4w+510 {\color{red}-510} = 1206 {\color{red}-510}\\ & 4w = {\color{red}696}\\ & \frac{4w}{{\color{red}4}} = \frac{696}{{\color{red}4}}\\ & \frac{\cancel{4}w}{\cancel{4}} = \frac{\overset{{\color{red}174}}{\cancel{696}}}{\cancel{4}}\\ & \boxed{w = 174}\\ \\ & l = w+255 && \text{Substitute the value for} \ w \ \text{into the equation.}\\ & l = {\color{red}174}+255 && \text{Solve the equation.}\\ & \boxed{l = 429}\end{align*}

The length of the rectangular plot of land is 429 yards and the width is 174 yards.

#### Solve using system of equations

Maria had $12100 to invest. She decided to invest her money in bonds and mutual funds. She invested a portion of the money in bonds paying 8% interest per year and the remainder in a mutual fund paying 9% per year. After one year the total income she had earned from the investments was$1043. How much had she invested at each rate?

The two quantities in this problem are the amount she had invested in bonds and the amount she had invested in mutual funds.

• Let the amount invested in bonds be represented by ‘\begin{align*}b\end{align*}’.
• Let the amount invested in mutual funds be represented by ‘\begin{align*}m\end{align*}’.

The equations would be:

• The total amount of money she had to invest was \begin{align*}\12,100 \rightarrow \boxed{b+m=12,100}\end{align*}
• The amount of money she earned from the investments was \begin{align*}\1043 \rightarrow \boxed{0.08b+0.09m=1043}\end{align*}

The system of equations will be solved using elimination.

\begin{align*}\begin{Bmatrix} b+m=12,100\\ 0.08b+0.09m=1043 \end{Bmatrix}\end{align*}

The first equation will be multiplied by (–0.08)

\begin{align*}& {\color{red}-0.08} (b+m=12,100)\\ & {\color{red}-0.08b-0.08m=-968}\\ & -0.08b-0.08m=-968\end{align*}

The equations now have opposite, numerical coefficients for the variable \begin{align*}b\end{align*}. Add the equations to eliminate the variable \begin{align*}b\end{align*}.

\begin{align*} & -{\cancel{0.08b}}-0.08m=-968\\ & \underline{\;\;\;\;\cancel{0.08b}+0.09m=1043\;} && \text{Solve the equation.}\\ & {\color{red}0.01m=75}\\ & 0.01m = 75\\ & \frac{0.01m}{{\color{red}0.01}} = \frac{75}{{\color{red}0.01}}\\ & \frac{\cancel{0.01}m}{\cancel{0.01}} = \frac{\overset{{\color{red}7500}}{\cancel{75}}}{\cancel{0.01}}\\ & \boxed{m = 7500}\\ \\ & b+m = 12,100 && \text{Substitute the value for} \ m \ \text{into the equation.}\\ & b+ {\color{red}7500} = 12,100 && \text{Solve the equation.}\\ & b+7500 {\color{red}-7500} = 12,100 {\color{red}-7500}\\ & \boxed{b = 4600}\end{align*}

Maria invested $4600 in bonds and$7500 in mutual funds.

### Examples

#### Example 1

Earlier, you were given a problem about two numbers.

Determine two numbers such that the sum of the numbers is 763 and the difference of the same two numbers is 179.

• Let ‘\begin{align*}x\end{align*}’ represent the larger number.
• Let ‘\begin{align*}y\end{align*}’ represent the smaller number.

Write an equation to model – the sum of two numbers is 763. \begin{align*}\rightarrow \boxed{x+y=763}\end{align*}

Write an equation to model – the difference of two numbers is 179. \begin{align*}\rightarrow \boxed{x-y=179}\end{align*}

The system of equations \begin{align*}\begin{Bmatrix} x+y=763\\ x-y=179 \end{Bmatrix}\end{align*} can be solved by a method of your choice. Choose one of the algebraic methods that you have learned to solve the system. Elimination would be a good choice since the variable ‘\begin{align*}y\end{align*}’ has opposite numerical coefficients.

\begin{align*}& \begin{Bmatrix} x+y=763\\ x-y=179 \end{Bmatrix}\\ \\ & x + {\cancel{y}}=763\\ & \underline{x -{\cancel{y}}=179} && \text{Add the equations to eliminate} \ y.\\ & \quad 2x=942\\ \\ & 2x=942 && \text{Solve the equation for the variable} \ x.\\ & \frac{2x}{2}=\frac{942}{2}\\ & \frac{\cancel{2}x}{\cancel{2}}=\frac{\overset{{\color{red}471}}{\cancel{942}}}{\cancel{2}}\\ & \boxed{x=471}\\ \\ & x+y=763 && \text{Substitute the value for} \ x \ \text{into one of the original equations.}\\ & ({\color{red}471})+y=763\\ & 471 {\color{red}-471}+y=763 {\color{red}-471} && \text{Solve the equation.}\\ & \boxed{y = 292}\end{align*}

The larger number is 471 \begin{align*}(x)\end{align*} and the smaller number is 292 \begin{align*}(y)\end{align*}.

#### Example 2

The sum of Henry’s and his mother’s age is 67. Three times Henry’s age increased by 7 is his mother’s age. How old is Henry?

The two quantities in this problem are Henry’s age and mother’s age. Let Henry’s age be represented by ‘\begin{align*}h\end{align*}’. Let mother’s age be represented by ‘\begin{align*}m\end{align*}’. The equations would be:

The sum of Henry’s age and his mother’s age is 67. \begin{align*}\rightarrow \boxed{h+m=67}\end{align*}
Three times Henry‘s age increased by 7 is his mother’s age \begin{align*}\rightarrow 3h+7=m\end{align*}
The system of equations will be solved by substitution.
\begin{align*}& \begin{Bmatrix} h+m=67\\ 3h+7=m \end{Bmatrix}\\ & h+m = 67 && \text{Substitute} \ 3h+7 \ \text{into the equation for} \ m.\\ & h+({\color{red}3h+7}) = 67 && \text{Apply the distributive property.}\\ & h {\color{red}+3h+7} = 67 && \text{Solve the equation.}\\ & {\color{red}4h}+7 = 67 &&\\ & 4h+7 {\color{red}-7} = 67 {\color{red}-7}\\ & 4h = {\color{red}60}\\ & \frac{4h}{{\color{red}4}} = \frac{60}{{\color{red}4}}\\ & \frac{\cancel{4}h}{\cancel{h}} = \frac{\overset{{\color{red}15}}{\cancel{60}}}{\cancel{4}}\\ & \boxed{h = 15} && \text{Henry is} \ 15 \ \text{years old. The problem asked for his mother's age.}\\ \\ & 3h+7 = m && \text{Substitute} \ 15 \ \text{into the equation for} \ h.\\ & 3({\color{red}15})+7 = m && \text{Solve the equation.}\\ & {\color{red}45}+7 = m\\ & \boxed{52 = m} && \text{Henry's mother is} \ 52 \ \text{years of age.}\end{align*}

#### Example 3

The Sydney Schooners played a total of 41 games of hockey. The number of games lost was ten less than one-half the number of games won. How many games did the Schooners win?

The two quantities in this problem are the games won and the games lost. Let games won be represented by ‘\begin{align*}w\end{align*}’. Let games lost be represented by ‘\begin{align*}l\end{align*}’. The equations would be:

The total number of games played (losses and wins) is 41. \begin{align*}\rightarrow \boxed{l+w=41}\end{align*}
The number of games lost was 10 less than one-half the number of games won. \begin{align*}\rightarrow \boxed{l=\frac{w}{2}-10}\end{align*}
The system of equations will be solved by elimination.
\begin{align*}& l = \frac{w}{2}-10 && \text{Multiply the equation by} \ 2.\\ & {\color{red}2}(l) = {\color{red}2} \left[\frac{w}{2}\right]-{\color{red}2}(10)\\ & {\color{red}2}(l) = {\color{red}\cancel{2}} \left[\frac{w}{\cancel{2}}\right]-{\color{red}2}(10)\\ & {\color{red}2l} = {\color{red}w}-{\color{red}20} && \text{Align the equations such that} \begin{Bmatrix} a_1 {\color{red}x}+b_1 {\color{blue}y}=c_1\\ a_2 {\color{red}x}+b_2 {\color{blue}y}=c_2 \end{Bmatrix}\\ & 2l {\color{red}-w} = w {\color{red}-w}-20\\ & \boxed{2l-w = -20}\end{align*}
The system of linear equations to solve by elimination is:
\begin{align*}& \begin{Bmatrix} l+w=41\\ 2l-w=-20 \end{Bmatrix}\\ \\ & l + \cancel{w} = 41\\ & \underline{2l -\cancel{w}=-20} && \text{Add the equations to eliminate} \ w.\\ & {\color{red}3l = 21}\\ & 3l=21 && \text{Solve the equation.}\\ & \frac{3l}{{\color{red}3}} = \frac{21}{{\color{red}3}}\\ & \frac{\cancel{3}l}{\cancel{3}} = \frac{\overset{{\color{red}7}}{\cancel{21}}}{\cancel{3}}\\ & \boxed{l=7} && \text{The number of games the Schooners lost was seven.}\\ \\ & l+w=41 && \text{Substitute} \ 7 \ \text{into the equation for the variable} \ l.\\ & {\color{red}7}+w=41 && \text{Solve the equation.}\\ &7 {\color{red}-7}+w=41 {\color{red}-7}\\ & w = {\color{red}34}\\ & \boxed{w=34} && \text{The number of games the Schooners won was} \ 34.\end{align*}

#### Example 4

Tim invested $3000. A portion of the money was invested into a college fund that paid 8% interest per year and the remainder was invested into a retirement fund that paid 7 % interest per year. At the end of the first year, the interest from the college fund was$60 more than the interest from the retirement fund. How much money did Tim invest in each fund?

The two quantities in this problem are the amount of money invested in the college fund and the amount of money invested in the retirement fund. Let the amount of money invested in the college fund be represented by ‘\begin{align*}c\end{align*}’. Let the amount of money invested in the retirement fund be represented by ‘\begin{align*}r\end{align*}’. The equations would be:

The total amount of money to be invested is 3000 \begin{align*}\rightarrow \boxed{c+r=3000}\end{align*} The amount of interest from the 8% college fund is60 more than the amount of interest from the 7% retirement fund. \begin{align*}\rightarrow \boxed{.08c=.07r+60}\end{align*}
The system of equations will be solved by substitution.
\begin{align*}& c+r = 3000 && \text{Solve the equation for} \ c.\\ & c+r {\color{red}-r} = 3000 {\color{red}-r}\\ & \boxed{c = 3000-r}\\ & 0.08c = 0.07r+60 && \text{Substitute} \ 3000-r \ \text{into the equation for the variable} \ c.\\ & 0.08({\color{red}3000-r}) = 0.07r+60 && \text{Apply the distributive property. Solve the equation.}\\ & {\color{red}240-0.08r} = 0.07r+60\\ & 240 {\color{red}-240}-0.08r = 0.07r+60 {\color{red}-240}\\ & -0.08r {\color{red}-0.07r} = 0.07r {\color{red}-0.07r}-180\\ & {\color{red}-0.15r} = -180\\ & \frac{-0.15r}{{\color{red}-0.15}} = \frac{-180}{{\color{red}-0.15}}\\ & \frac{\cancel{-0.15}r}{\cancel{-0.15}} = \frac{\overset{{\color{red}1200}}{\cancel{-180}}}{\cancel{-0.15}}\\ & \boxed{r = 1200} && \text{The amount of money invested in the retirement fund was} \ \1200.\\ \\ & c = 3000-r && \text{Substitute} \ 1200 \ \text{into the equation for} \ r.\\ & c = 3000 -{\color{red}1200} && \text{Solve the equation.}\\ & c = {\color{red}1800}\\ & \boxed{c = 1800} && \text{The amount of money invested in the college fund was} \ \1800.\end{align*}

### Review

Write each of the following statements as a linear equation in two variables.

1. The sum of two numbers is 100.
2. When six times the larger of two numbers is added to 4 times the smaller, the result is 112.
3. The length of a rectangle is 8m more than 7 times its width.
4. Four times the number of nickels less three times the number of pennies is 56.
5. Jason has some $5 bills and some$1 bills which have a total of 91. Solve each of the following problems using \begin{align*}2 \times 2\end{align*} systems of linear equations. 1. At a party, there were 72 people. The hostess counted the shoes and found that there were 32 more pairs of ladies’ shoes compared to the number of pairs of males’ shoes. How many males and females were at the party? 2. Two weight loss programs offer competitive services. Super Slim charges33 to join and $1.50 per session whereas Think Thin charges$2.50 per session and $15 to join. Determine algebraically, under what circumstances you would choose each plan. 3. Today Sam is twice Jenny’s age. Three years ago the sum of their ages was 45. How old are Sam and Jenny today? 4. The parking lot at a local amusement park contained 123 vehicles (cars and buses). Each car is charged$3 to park for the day and each bus is charged $10. If the total revenue for the day was$481.00, how many cars were on the parking lot?
5. Seven times the larger of two numbers less three times the smaller is 351. Six times the larger less twice the smaller is 342. What are the numbers?
6. The debate team washed cars to raise money for a trip. They charged $8 for a large car and$5 for a small car. All together they raised $550 and washed 80 cars. How many of each type of car did they wash? 7. Pencils cost$0.10 each and notebooks cost $2 each. You buy 15 items and spend$9.10. How many pencils did you buy? How many notebooks did you buy?
8. The sum of two numbers is 15 and the product of the same two numbers is 36. What are the numbers?
9. Cereal costs $3.50 a box and milk costs$2.79 a gallon. Suppose you buy five items and spend \$16.08. How many boxes of cereal did you buy and how many gallons of milk did you buy?
10. Twice the sum of two number is 72 while the difference between the two numbers is 22. What are the numbers?

To see the Review answers, open this PDF file and look for section 5.4.

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Color Highlighted Text Notes

### Vocabulary Language: English

elimination

The elimination method for solving a system of two equations involves combining the two equations in order to produce one equation in one variable.

linear equation

A linear equation is an equation between two variables that produces a straight line when graphed.

Linear System of Equation(s)

A linear system of equations is a set of equations that must be solved together to find the one solution that fits them both.

system of equations

A system of equations is a set of two or more equations.

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