Question

A recipe calls for 3 cups of water per cup of grape juice. How much grape juice for 21 cups of water?

Answer 1

For every cup of grape juice, you have to use 3 cups of water.

To determine how many cups of grape juice you have to use for 21 cups of water, you can use cross multiplication:

3 cups of water____ 1 cup of juice

21 cups of water____ x cups of juice

Both relationships are at the same ratio so that:

$\frac{1}{3}=\frac{x}{21}$

Multiply both sides by 21 to determine the value of x:

$\begin{gathered} 21\cdot\frac{1}{3}=21\cdot\frac{x}{21} \\ 7=x \end{gathered}$

For 21 cups of water, you have to use 7 cups of grape juice.