Answer
See below.
Work Step by Step
We know that $probability=\frac{\text{number of favourable outcomes}}{\text{number of all outcomes}}$
a) The number of favourable outcomes is $1$, the number of all outcomes is ${20\choose5}=15504$, hence the probability is: $\frac{1}{15504}$
a) The number of favourable outcomes is $100$, the number of all outcomes is ${20\choose5}=15504$, hence the probability is: $\frac{100}{15504}=\frac{25}{3876}$
