Imagine that you know the value of $50$ chocolate bars, and someone asks you what the value of $13$ chocolate bars is. In that case, it’s smart to find the cost per unit, which is the value of $1$ single chocolate bar. This recipe shows you how to do that:

Rule

- 1.
- To find the cost per unit, divide the total value by the number of units that make up that value.
- 2.
- Then you can multiply the cost per unit you found in Item 1 by the number of units you want to find the value of.

Example 1

**A magnum bottle of champagne costs $\text{\$}\text{}350\text{}$ and contains 25 glasses of champagne. A group of girls bought one bottle, but gave away four glasses to the clingy boys close by. How much did that cost the girls?**

In problems like this one, it is helpful to figure out the cost per unit, which in this case is the price of one glass of champagne. Then you can multiply that price by the $4$ glasses the girls gave away to find out how much they lost.

The price of one glass of champagne is $$\begin{array}{llllll}\hfill \text{\$}350& =\text{25glasses}\phantom{\rule{2em}{0ex}}& \hfill & \phantom{\rule{2em}{0ex}}& \hfill & \phantom{\rule{2em}{0ex}}\\ \hfill \text{\$}350& =\text{25glasses}\phantom{\rule{2em}{0ex}}& \hfill & \phantom{\rule{1em}{0ex}}|:25\phantom{\rule{2em}{0ex}}& \hfill & \phantom{\rule{2em}{0ex}}& \hfill & \phantom{\rule{2em}{0ex}}\\ \hfill \frac{\text{\$}350}{25}& =\frac{25}{25}\text{glasses}\phantom{\rule{2em}{0ex}}& \hfill & \phantom{\rule{2em}{0ex}}& \hfill & \phantom{\rule{2em}{0ex}}\\ \hfill \text{\$}1.40& =\text{1glass}.\phantom{\rule{2em}{0ex}}& \hfill & \phantom{\rule{2em}{0ex}}& \hfill & \phantom{\rule{2em}{0ex}}\end{array}$$

Then the price of four glasses is

$$1.40\cdot 4=\text{\$}5.60.$$ |

Giving away four glasses of champagne cost the girls a total of $$5.60$.

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