What if you had two types of grape drink: one with 5% real fruit juice and another with 10% real fruit juice? Suppose you wanted a gallon of grape drink with 6% real fruit juice. How much of the 5% drink and how much of the 10% drink should you mix together to produce it? In this Concept, you'll learn how to solve mixture problems like this one.

### Guidance

Systems of equations arise in chemistry when mixing chemicals in solutions and can even be seen in things like mixing nuts and raisins or examining the change in your pocket!

By rearranging one sentence in an equation into *algebraic expression* or *algebraic expression*, you can use the Substitution Method to solve the system.

#### Example A

*Nadia empties her purse and finds that it contains only nickels and dimes. If she has a total of 7 coins and they have a combined value of 55 cents, how many of each coin does she have?*

Solution: Begin by choosing appropriate variables for the unknown quantities. Let *the number of nickels* and *the number of dimes*.

There are seven coins in Nadia’s purse:

The total is 55 cents:

The system is:

We can quickly rearrange the first equation to isolate

Using the Substitution Property, every

Nadia has 3 nickels. There are seven coins in the purse; three are nickels, so four must be dimes.

Check to make sure this combination is 55 cents:

#### Example B

*A chemist has two containers, Mixture A and Mixture B. Mixture A has a 60% copper sulfate concentration. Mixture B has a 5% copper sulfate concentration. The chemist needs to have a mixture equaling 500 mL with a 15% concentration. How much of each mixture does the chemist need?*

Solution: Although not explicitly stated, there are two equations involved in this situation.

- Begin by stating the variables. Let
A=mixture A and B=mixture B . - The total mixture needs to have 500 mL of liquid.

Equation 1 (how much total liquid):

- The total amount of copper sulfate needs to be 15% of the total amount of solution (500 mL).
0.15⋅500=75 ounces

Equation 2 (how much copper sulfate the chemist needs):

By rewriting equation 1, the Substitution Property can be used:

Substitute the expression

Solve for

The chemist needs approximately 409 mL of mixture

The chemist needs 91 milliliters of mixture

#### Example C

*A coffee company makes a product which is a mixture of two coffees, using a coffee that costs $10.20 per pound and another coffee that costs $6.80 per pound. In order to make 20 pounds of a mixture that costs $8.50 per pound, how much of each type of coffee should it use?*

**Solution:**

Let

Also, the amount of each type of coffee added together equals 20 pounds:

The system is:

We can isolate one variable and use substitution to solve the system:

Since

The coffee company needs to use 10 pounds of each type of coffee in order to have a 20 pound mixture that costs $8.50 per pound.

### Guided Practice

*A light green latex paint that is 20% yellow paint is combined with a darker green latex paint that is 45% yellow paint. How many gallons of each paint must be used to create 15 gallons of a green paint that is 25% yellow paint?*

**Solution:**

Let

Now if we want 15 gallons of 25% yellow paint, that means we want

\begin{align*} 0.20x+0.40y=3.75\end{align*}

The system is: \begin{align*}\begin{cases} \qquad \quad \ \ x+y=15\\ 0.20x+0.45y=3.75 \end{cases}\end{align*}.

We can isolate one variable and use substitution to solve the system:

\begin{align*}x=15-y\end{align*}

\begin{align*}0.20(15-y)+0.45y&=3.75\\ \text{Now solve for} \ x: \qquad 3-0.20y+0.45y&=3.75 \qquad \text{Distributive Property}\\ 3+0.2y&=3.75 \qquad \text{Add like terms.}\\ 0.25y&=0.75\quad \ \text{Subtract} \ 3.\\ y& =3 \qquad \quad \ \text{Divide by} \ 0.25.\end{align*}

Now we can plug in \begin{align*} y=3 \end{align*} into \begin{align*} x+y=15\end{align*}:

\begin{align*}x+y=15 \Rightarrow x+3=15 \Rightarrow x=12\end{align*}.

This means 12 gallons of 20% yellow paint should be mixed with 3 gallons of 45% yellow paint in order to get 15 gallons of 25% yellow paint.

### Practice

- I have $15.00 and wish to buy five pounds of mixed nuts for a party. Peanuts cost $2.20 per pound. Cashews cost $4.70 per pound. How many pounds of each should I buy?
- A chemistry experiment calls for one liter of sulfuric acid at a 15% concentration, but the supply room only stocks sulfuric acid in concentrations of 10% and 35%. How many liters of each should be mixed to give the acid needed for the experiment?
- Bachelle wants to know the density of her bracelet, which is a mix of gold and silver. Density is total mass divided by total volume. The density of gold is 19.3 g/cc and the density of silver is 10.5 g/cc. The jeweler told her that the volume of silver used was 10 cc and the volume of gold used was 20 cc. Find the combined density of her bracelet.
- Jeffrey wants to make jam. He needs a combination of raspberries and blackberries totaling six pounds. He can afford $11.60. How many pounds of each berry should he buy?
- A farmer has fertilizer in 5% and 15% solutions. How much of each type should he mix to obtain 100 liters of fertilizer in a 12% solution?

**Mixed Review**

- The area of a square is \begin{align*}96 \ inches^2\end{align*}. Find the length of a square exactly.
- The volume of a sphere is \begin{align*}V= \frac{4}{3} \pi r^3\end{align*}, where \begin{align*}r=radius\end{align*}. Find the volume of a sphere with a diameter of 11 centimeters.
- Find:
- the additive inverse of 7.6
- the multiplicative inverse of 7.6.

- Solve for \begin{align*}x: \frac{1.5}{x}=6\end{align*}.
- The temperature in Fahrenheit can be approximated by crickets using the rule “Count the number of cricket chirps in 15 seconds and add 40.”
- What is the domain of this function?
- What is the range?
- Would you expect to hear any crickets at \begin{align*}32^\circ F\end{align*}? Explain your answer.
- How many chirps would you hear if the temperature were \begin{align*}57^\circ C\end{align*}?

- Is 4.5 a solution to \begin{align*}45-6x \le 18\end{align*}?
- Graph \begin{align*}y=|x|-5\end{align*}.
- Solve. Write the solution using interval notation and graph the solution on a number line: \begin{align*}-9\ge \frac{c}{-4}\end{align*}.
- The area of a rectangle is 1,440 square centimeters. Its length is ten times more than its width. What are the dimensions of the rectangle?
- Suppose \begin{align*}f(x)=8x^2-10\end{align*}. Find \begin{align*}f(-6)\end{align*}.
- Torrey is making candles from beeswax. Each taper candle needs 86 square inches and each pillar candle needs 264 square inches. Torrey has a total of 16 square feet of beeswax. Graph all the possible combinations of taper and pillar candles Torrey could make (Hint: \begin{align*}one \ square \ foot=144 \ square \ inches\end{align*}).