With five houses every three hours, Brianna will paint 20 houses in 12 hours. 12 hours / 3 hours = 4 'rounds'. 5 houses * 4 rounds = 20 houses. With three houses every four hours, Nicholas will paint 9 houses in 12 hours. 12 hours / 4 hours = 3 'rounds'. 3 houses * 3 rounds = 9 houses. So Brianna will paint 20 - 9 more houses than Nicholas, or 11 houses more.