This is a two parts question.
Firstly we want to know the amount of commission made on sales of $5,000 worth water
From the question, we are told that he gets 15% commission on his water sales; this means that the amount made from the sales will be;
15% of $5,000
= 15/100 * 5,000 = 15 * 50 = $750
He made a commission of $750 0n a sales of $5,000 worth water
Secondly, after making a commission of $4,500 after the drought, we want to know the amount of water sold during the drought
Let the amount made be be $x
15% of this is $4,500
15/100 * x = 4,500
15x = 100 * 4,500
x = (100 * 4500)/15
x = $30,000
So for a commission of $4,500 , $30,000 worth of water was sold