In a batch of 30 apples, 1/6 are too small and 1/3 are not ripe.  The others are perfect. Calculate the number of perfect apples.
We first calculate the proportion of all the other apples: 1/6 + 1/4 + 1/12 = 2/12 + 3/12 + 1/12 =6/12
So, the proportion of perfect apples is 12/12 - 6/12 = 6/12 = 0.5.
Thus the number of perfect apples is 0.5 * 30 = <<0.5*30=15>>15
#### 15