What is the smallest integer k such that k! is a multiple of 2^20?

To do this, we see that 2^20 is also known as 20 2's. So, we start from 1, and keep on going until we get a total of more than 20 2's:

Number.         How may 2s

1                    0

2                    1

3                    0

4                   2

5                    0

6                  1

7                   0

8                  3

9                   0

10                1

11                  0

12                 2

13                0

...

keep on going until the sum of the collumn "How many 2s" is greater than 20!