Avat contains 60 gallons of a 15% saline solution. how many gallons of water must be evaporated so that the resulting solution will be 20% saline?

To solve this problem we can use proportion.
60 gallons -----------100%
x-------------------------15%
gallons of saline.
Now we can use this knowledge and create second proportion
9 galllons -------------20%
x-------------------------100%

galloons
Now we can just find the difference (evaporated gallons)
60-45=15 - its our result.