There are some method to multiply with 5,15,25,125 as -

to multiply with 5 = 10 / 2

to multiply with 15 = (x + x / 2)*10

to multiply with 25 = 100 / 4

to multiply with 125 = 1000 / 8

Q1. 2000 * 15

As mentioned above to multiply with 15 we multiply with (x+x/2)*10 so

2000 * 15 = (2000 + 2000 / 2)* 10

= 3000 * 10

= 30000