What number should replace the question mark? ? ; 15 ; 60 ; 180 ; 360

Question

What number should replace the question mark?

28 -> 9

52 -> 15

84 -> 23

20 -> 7

100 -> ?

Answer 1

3

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

Answer 2

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.