He will need 300 ounces of water because 100 x 3 = <<100*3=300>>300
He will need 6 bottles because 300 / 50 = <<300/50=6>>6
This will cost $15 because 6 x 2.5 = <<6*2.5=15>>15
He has $20 because 2 x 10 = <<2*10=20>>20
He will get $5 in change because 20 - 15 = <<20-15=5>>5
#### 5