The answer 20.
In the third row there are 4 whistles (2 in each pic), both sets of whistles equals 8, so since there is only 1 whistle in the last row it equals 2.
Depends if the single whistle is a mistake or not.
30/3 = Shoes (Shoes are 10 combined in a pair)
(20-10)/2= 10/2 = Hat Person (5 per person)
(13-5)/2 = 8/2 = Pair of Whistles (4 per pair, or 2 per whistle even if the pair is multiplied 2+2=2x2)
Shoes(Pair) + Hat Person x Single Whistle = ?
10 + 5 x 2 = ?
10 + 10 = ?
20 = ?
If this single whistle is a mistake the answer is 30.
Also noticing the hat person HAD a whistle on every panel except for the last iteration... So in that eventuality it becomes...