What if you had a jar of pennies? You take out a handful that is equal to

### Watch This

CK-12 Foundation: 0207S Real-World Multiplication and Division (H264)

### Guidance

Using the skills learned in the last two concepts, you now ready to solve real-world problems.

#### Example A

*In the chemistry lab there is a bottle with two liters of a 15% solution of hydrogen peroxide (H2O2). John removes one-fifth of what is in the bottle, and puts it in a beaker. He measures the amount of H2O2 and adds twice that amount of water to the beaker. Calculate the following measurements.*

a) The amount of

b) The amount of diluted

c) The concentration of the

**Solution**

a) To determine the amount of

The amount remaining is

There are 1.6 liters left in the bottle.

b) We determined that the amount of the 15%

There are 1.2 liters of diluted

c) The new concentration of

John started with **pure**

After he adds the water, there is 1.2 liters of solution in the beaker, so the concentration of

#### Example B

*Anne has a bar of chocolate and she offers Bill a piece. Bill quickly breaks off 14 of the bar and eats it. Another friend, Cindy, takes 13 of what was left. Anne splits the remaining candy bar into two equal pieces which she shares with a third friend, Dora. How much of the candy bar does each person get?*

First, let’s look at this problem visually.

Anne starts with a full candy bar.

Bill breaks off

Cindy takes

Dora gets half of the remaining candy bar.

We can see that the candy bar ends up being split four ways, with each person getting an equal amount.

**Solution**

Each person gets exactly

We can also examine this problem using rational numbers. We keep a running total of what fraction of the bar remains. Remember, when we read a fraction followed by *of* in the problem, it means we multiply by that fraction.

We start with 1 bar. Then Bill takes

Cindy takes

That half bar gets split between Anne and Dora, so they each get half of a half bar:

So each person gets exactly

**Extension:** If each person’s share is 3 oz, how much did the original candy bar weigh?

#### Example C

*Newton’s second law* *relates the force applied to a body in Newtons* *the mass of the body in kilograms* *and the acceleration in meters per second squared* *Calculate the resulting acceleration if a Force of* *Newtons is applied to a mass of*

**Solution**

First, we rearrange our equation to isolate the acceleration,

\begin{align*}a = \frac{7 \frac{1}{3}}{\frac{1}{5}} = \frac{22}{3} \div \frac{1}{5} = \frac{22}{3} \times \frac{5}{1} = \frac{110}{3}\end{align*}

The resultant acceleration is \begin{align*}36 \frac{2}{3} \ m/s^2\end{align*}.

Watch this video for help with the Examples above.

CK-12 Foundation: Solving Real-World Problems Using Multiplication and Division

### Guided Practice

*Andrew is driving down the freeway. He passes mile marker 27 at exactly mid-day. At 12:35 he passes mile marker 69. At what speed, in miles per hour, is Andrew traveling?*

**Solution**

To find the speed, we need the distance traveled and the time taken. If we want our speed to come out in miles per hour, we’ll need distance in **miles** and time in **hours**.

The distance is \begin{align*}69 - 27 \end{align*} or 42 miles. The time is 35 minutes, or \begin{align*}\frac{35}{60}\end{align*} hours, which reduces to \begin{align*}\frac{7}{12}\end{align*}. Now we can *plug in* the values for distance and time into our equation for speed.

\begin{align*}\text{Speed} = \frac{42}{\frac{7}{12}} = 42 \div \frac{7}{12} = \frac{42}{1} \times \frac{12}{7} = \frac{6 \cdot 7 \cdot 12}{1 \cdot 7} = \frac{6 \cdot 12}{1} = 72\end{align*}

Andrew is driving at 72 miles per hour.

### Explore More

For 1-8, perform the operations of multiplication and division.

- \begin{align*}(3 - x)\end{align*}
- \begin{align*}\frac{1}{13} \times \frac{1}{11}\end{align*}
- \begin{align*}\frac{7}{27} \times \frac{9}{14}\end{align*}
- \begin{align*}\left ( \frac{3}{5} \right )^2\end{align*}
- \begin{align*}\frac{1}{2} \div \frac{x}{4y}\end{align*}
- \begin{align*}\left ( - \frac{1}{3} \right ) \div \left( - \frac{3}{5} \right )\end{align*}
- \begin{align*}\frac{7}{2} \div \frac{7}{4}\end{align*}
- \begin{align*}11 \div \frac{-x}{4}\end{align*}

For 9-11, solve the real-world problems using multiplication and division.

- The label on a can of paint says that it will cover 50 square feet per pint. If I buy a \begin{align*}\frac{1}{8}\end{align*} pint sample, it will cover a square two feet long by three feet high. Is the coverage I get more, less or the same as that stated on the label?
- The world’s largest trench digger, "Bagger 288", moves at \begin{align*}\frac{3}{8}\end{align*} mph. How long will it take to dig a trench \begin{align*}\frac{2}{3}\end{align*} mile long?
- A \begin{align*}\frac{2}{7}\end{align*} Newton force applied to a body of unknown mass produces an acceleration of \begin{align*}\frac{3}{10} \ m/s^2\end{align*}. Calculate the mass of the body.

### Answers for Explore More Problems

To view the Explore More answers, open this PDF file and look for section 2.7.