3

3*5=15 15*4=60 60*3=180 180*2=360

The multiplying factor is in the descending order. 3 x 5 = 15 15 x 4 = 60 60 x 3 = 180 180 x 2 = 360

So the answer is 3.

What number should replace the question mark?

28 -> 9

52 -> 15

84 -> 23

20 -> 7

100 -> ?