What if Marisha's target heart rate is 0.70 times the quantity 220 minus her age. Her target heart rate is 140. What is her age? After completing this Concept, you'll be able to solve real-world applications like this one that involve two-step equations.

### Watch This

CK-12 Foundation: 0306S Solving Real World Problems with Two Step Equations (H264)

### Guidance

The hardest part of solving word problems is translating from words to an equation. First, you need to look to see what the equation is asking. What is the **unknown** for which you have to solve? That will be what your **variable** stands for. Then, follow what is going on with your variable all the way through the problem.

#### Example A

*An emergency plumber charges $65 as a call-out fee plus an additional $75 per hour. He arrives at a house at 9:30 and works to repair a water tank. If the total repair bill is $196.25, at what time was the repair completed?*

In order to solve this problem, we collect the information from the text and convert it to an equation.

**Unknown:** time taken in hours – this will be our \begin{align*}x\end{align*}

The bill is made up of two parts: a call out fee and a per-hour fee. The call out is a flat fee, and independent of \begin{align*}x\end{align*}—it’s the same no matter how many hours the plumber works. The per-hour part depends on the number of hours \begin{align*}(x)\end{align*}. So the total fee is $65 (no matter what) plus \begin{align*}\$75x\end{align*} (where \begin{align*}x\end{align*} is the number of hours), or \begin{align*}65 + 75x\end{align*}.

Looking at the problem again, we also can see that the total bill is $196.25. So our final equation is \begin{align*}196.25 = 65 + 75x\end{align*}.

Solving for \begin{align*}x\end{align*}:

\begin{align*}196.25 &= 65 + 75x \qquad \text{Subtract 65 from both sides.}\\ 131.25 &= 75x \qquad \qquad \text{Divide both sides by 75.}\\ 1.75 &= x \qquad \qquad \quad \text{The job took 1.75 hours.}\end{align*}

**Solution**

The repair job was completed 1.75 hours after 9:30, so it was completed at 11:15AM.

#### Example B

*When Asia was young her Daddy marked her height on the door frame every month. Asia’s Daddy noticed that between the ages of one and three, he could predict her height (in inches) by taking her age in months, adding 75 inches and multiplying the result by one-third. Use this information to answer the following:*

a) *Write an equation linking her predicted height, \begin{align*}h\end{align*}, with her age in months, \begin{align*}m\end{align*}.*

b) *Determine her predicted height on her second birthday.*

c) *Determine at what age she is predicted to reach three feet tall.*

**Solution**

a) To convert the text to an equation, first determine the type of equation we have. We are going to have an equation that links **two variables**. Our unknown will change, depending on the information we are given. For example, we could solve for height given age, or solve for age given height. However, the text gives us a way to determine **height**. Our equation will start with “\begin{align*}h=\end{align*}”.

The text tells us that we can predict her height by taking her age in months, adding 75, and multiplying by \begin{align*}\frac{1}{3} \end{align*}. So our equation is \begin{align*}h = (m + 75) \cdot \frac{1}{3} \end{align*}, or \begin{align*}h = \frac{1}{3}(m + 75)\end{align*}.

b) To predict Asia’s height on her second birthday, we substitute \begin{align*}m=24\end{align*} into our equation (because 2 years is 24 months) and solve for \begin{align*}h\end{align*}.

\begin{align*} h &= \frac{1}{3}(24 + 75)\\ h &= \frac{1}{3}(99)\\ h &= 33\end{align*}

Asia’s height on her second birthday was predicted to be 33 inches.

c) To determine the predicted age when she reached three feet, substitute \begin{align*}h = 36\end{align*} into the equation and solve for \begin{align*}m\end{align*}.

\begin{align*}36 &= \frac{1}{3}(m + 75)\\ 108 &= m + 75\\ 33 &= m\end{align*}

Asia was predicted to be 33 months old when her height was three feet.

#### Example C

*To convert temperatures in Fahrenheit to temperatures in Celsius, follow the following steps: Take the temperature in degrees Fahrenheit and subtract 32. Then divide the result by 1.8 and this gives the temperature in degrees Celsius.*

a) *Write an equation that shows the conversion process.*

b) *Convert 50 degrees Fahrenheit to degrees Celsius.*

a) The text gives the process to convert Fahrenheit to Celsius. We can write an equation using two variables. We will use \begin{align*}f\end{align*} for temperature in Fahrenheit, and \begin{align*}c\end{align*} for temperature in Celsius.

\begin{align*}&\text{First we take the temperature in Fahrenheit and subtract 32.} && f - 32\\ &\text{Then divide by 1.8.} && \frac{f - 32}{1.8}\\ &\text{This equals the temperature in Celsius.} && c = \frac{f - 32}{1.8}\end{align*}

In order to convert from one temperature scale to another, simply substitute in for whichever temperature you know, and solve for the one you don’t know.

b) To convert 50 degrees Fahrenheit to degrees Celsius, substitute \begin{align*}f = 50\end{align*} into the equation.

\begin{align*}c &= \frac{50 - 32}{1.8}\\ c &= \frac{18}{1.8}\\ c &= 10\end{align*}

50 degrees Fahrenheit is equal to 10 degrees Celsius.

Watch this video for help with the Examples above.

CK-12 Foundation: Solving Real-World Problems with Two-Step Equations

### Vocabulary

- Some equations require more than one operation to solve. Generally it, is good to go from the outside in. If there are parentheses around an expression with a variable in it, cancel what is outside the parentheses first.
- Terms with the same variable in them (or no variable in them) are
**like terms. Combine like terms**(adding or subtracting them from each other) to simplify the expression and solve for the unknown.

### Guided Practice

a) *Convert 25 degrees Celsius to degrees Fahrenheit.*

b) *Convert -40 degrees Celsius to degrees Fahrenheit.*

**Solution:**

a) To convert 25 degrees Celsius to degrees Fahrenheit, substitute \begin{align*}c = 25\end{align*} into the equation:

\begin{align*} 25 &= \frac{f - 32}{1.8}\\ 45 &= f - 32\\ 77 &= f \end{align*}

25 degrees Celsius is equal to 77 degrees Fahrenheit.

b) To convert -40 degrees Celsius to degrees Fahrenheit, substitute \begin{align*}c = -40\end{align*} into the equation.

\begin{align*}-40 &= \frac{f - 32}{1.8}\\ -72 &= f - 32\\ -40 &= f\end{align*}

-40 degrees Celsius is equal to -40 degrees Fahrenheit. (No, that’s not a mistake! This is the one temperature where they are equal.)

### Practice

- Jade is stranded downtown with only $10 to get home. Taxis cost $0.75 per mile, but there is an additional $2.35 hire charge. Write a formula and use it to calculate how many miles she can travel with her money.
- Jasmin’s Dad is planning a surprise birthday party for her. He will hire a bouncy castle, and will provide party food for all the guests. The bouncy castle costs $150 for the afternoon, and the food will cost $3 per person. Andrew, Jasmin’s Dad, has a budget of $300. Write an equation and use it to determine the maximum number of guests he can invite.

The local amusement park sells summer memberships for $50 each. Normal admission to the park costs $25; admission for members costs $15.

- If Darren wants to spend no more than $100 on trips to the amusement park this summer, how many visits can he make if he buys a membership with part of that money?
- How many visits can he make if he does not?
- If he increases his budget to $160, how many visits can he make as a member?
- And how many as a non-member?

For an upcoming school field trip, there must be one adult supervisor for every five children.

- Write an expression for the number of children and adults who will go on the field trip.
- If the bus seats 40 people, how many children can go on the trip?
- How many children can go if a second 40-person bus is added?
- Four of the adult chaperones decide to arrive separately by car. Now how many children can go in the two buses?