He needs 1 can of frosting per layer cake and he needs to make 3 layer cakes so that's 1*3 = <<1*3=3>>3 cans of frosting
There are 6 dozen cupcakes,12 single cakes and 18 pans of brownies for a total of 6+12+18 = <<6+12+18=36>>36 orders
Each of the 36 orders needs 1/2 can of frosting so they need 36*.5 = <<36*.5=18>>18 cans of frosting
He needs 3 cans of frosting for the layer cakes and 18 cans for the remaining items for a total of 3+18 = <<3+18=21>>21 cans of frosting
#### 21