At the circus, the clown has 3 dozen balloons on a string in his hand.  3 boys and 12 girls buy a balloon each.  How many balloons is the clown still holding?
The clown is holding 3 x 12 = <<3*12=36>>36 balloons.
This many children buy a balloon: 3 + 12 = <<3+12=15>>15 kids.
After the children buy the balloons, the clown is holding this many in his hand 36 - 15 = <<36-15=21>>21 balloons.
#### 21