Grant plans to evaporate enough water from 22 gallons of a 16% ammonia solution to make a 24% ammonia solution. which equation can he use to find n, the number of gallons of water he should remove?

First lets write down our starting conditions:

we have 22 gallons and it has 16% of ammonia

16% of 22 gallons is
A = 16/100*22 = 3,52 gallons

Now since only watter is evaporating that means that in second scenario after some watter evaporated there is still 3,52 gallons of ammonia. But now those 3,52 gallons of ammonia represent 24% of total mixture. So now we go other way. 
24 % is 3,52 gallons
100% is x gallons

x = 100/24*3,52 = 14.667 gallons

That means that he has to evaporate 
22 - 14.6667 = 7,333 gallons of water.