Replace Question Mark?

Replace Question Mark?

Answer is: 15

It really simple Question! :D
8-80 (8 x 10 = 80)
then
9-45 (9 x 5 = 45)
then
2-20 (2 x 10= 20)
so next in this series is
3-? (3 x 5= 15)
so we will write 3-15