Calculating subnet mask

The formula for finding out the number of bits is:

(2^n) – 2 = x

Where “n” is the number of bits you need to borrow and “x” is the number of subnets that will be produced.

You need 5 subnets:

(2^n) – 2 = x

Try 2 first:
(2^2) = 4 and 4-2 is 2 (this is too low)

Try 3 next:
(2^3) = 8 and 8-2 is 6 (this is correct since it will give you 6 subnets)

You have borrowed 3 bits.
So for your example, you would end up with:

199.120.16.0 /21 (since 24-3 = 21)