10 + 10 + 10 = 30
10 + 5 + 5 = 20
5 + 4 + 4 = 13
10 + (3 x 2) = 16

Explanation: in line 3 each set of two whistles is equal to 4, which means that each whistle is worth 2.  In line 4 the girl is not wearing a whistle around her neck (like she was in lines 2 and 3) so you subtract the value of one whistle from the value of the girl.  So it ends up being 10 (pair of shoes) + ( 3 (girl minus the Whistle value) x 2 (one whistle)) 

10 + (3 x 2) 
10 + 6
16