Answer
See below.
Work Step by Step
We know that $probability=\frac{\text{number of favorable outcomes}}{\text{number of all outcomes}}$
a) The number of all outcomes is $_{20}C_5=15504$, out of these $1$ corresponds to the desirable selection. Hence here the probability is $\frac{1}{15504}\approx0.0000645 $
b) The number of all outcomes is $_{20}C_5=15504$, out of these $100$ correspond to the desirable selection. Hence here the probability is $\frac{100}{15504}\approx0.00645 $