Question

If 1 equals with 2, 4 equals with 20, and 9 equals with 90, then 10 equals with what?

Answer 1

1 => 1 * (1 + 1) => 2
4 => 4 * (4 + 1) => 20

Hint.

Answer 2

2=1(1+1)
20=4(4+1)
90=9(9+1)
so
for 10,
10(10+1)=answer!