Answer
See below.
Work Step by Step
The odds for $E$: $\frac{P(E)}{P(\text{not E})}$. The probability for $E$: $\frac{P(E)}{P(\text{not E})+P(E)}$. The odds against $E$: $\frac{P(\text{not E})}{P(E)}$ The probability against $E$: $\frac{P(\text{not E})}{P(\text{not E})+P(E)}$.
Hence here the probability for making it is: $\frac{21}{21+4}=\frac{21}{25}$, thus the probability of missing it: $1-\frac{21}{25}=\frac{4}{25}$.
Out of $100$ shots on average he made: $100\cdot\frac{21}{25}=4\cdot21=84$.
