To solve this problem we can use proportion.
60 gallons -----------100%
x-------------------------15%
[tex]x= \frac{60*15}{100} = 9[/tex] gallons of saline.
Now we can use this knowledge and create second proportion
9 galllons -------------20%
x-------------------------100%
[tex]x= \frac{9*100}{20}=45 [/tex] galloons
Now we can just find the difference (evaporated gallons)
60-45=15 - its our result.
