Diego baked 12 cakes for his sister's birthday. Donald also baked 4 cakes, but ate 1 while waiting for the party to start. How many cakes are left?
The total number of cakes baked is 12 + 4 = <<12+4=16>>16 cakes.
But since Diego ate 1 cake, there are 16 - 1 = <<16-1=15>>15 cakes left.
#### 15