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:

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

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