how to multiply1111X101 in base 2 binary

Answer 1

1111 * 101
(1*2^3+1*2^2+1*2^1+1*2^0)*(1*2^2+0*2^1+1*2^0) = (8+4+2+1)*(4+0+1)
= (15) * (5) = 75

Answer 2

Correction: You probably want the multiplication done without converting to base 10. I can't show it clearly here
But it is long-hand multiplication.

1111
0101

0011110
0111100
1001011= Product = 75 base 10.