Drawing numbers from a set, quantile We have a set containing $20$ numbers from $1$ to $20$. Each time we draw only one number, and repeat it $15$ times (without replacement). Let's denote $X-$ the largest drawn number. Find the smallest ${{16} \choose {15}}/{20 \choose 15}$-fractile  of a random variable $X$.
So, we look for such $x_p$ that $F(x_p)\geq {{16} \choose {15}}/{20 \choose 15}.$ If $X$ is maximum of all drawn numbers, then $F_X(t)=\Bbb P(X\leq t)=\Bbb P(X_1\leq t, ...,X_m\leq t)$ and, if all of $X_1,...X_m$ are iid then we have $\Bbb P(X_1\leq t)\cdot...\cdot\Bbb P(X_m\leq t)$. But... how to proceed?

There are $\binom {20} {15}$ ways to choose 15 numbers from the set.
$\binom{16}{15}$ is the number of ways to choose 15 numbers only from $1$ to $16$ inclusive. The maximum of such subset is at most $16$. So the cumulative probability up to and including $t=16$ is the $\binom {16} {15} / \binom {20} {15}$.

Alternatively, consider breaking down to individual probabilitites $\Pr(X=t)$, where $15\le t \le 20$.
To choose a subset with a maximum of $t$, the subset will have 14 elements between $1$ and $t-1$. The number of subsets with a maximum $t$ is $\binom {t-1}{14}$.
Verify that the total number of subset is $\binom {20}{15}$: (the hockey-stick identity)
$$\sum_{t=15}^{20}\binom{t-1}{14} = \sum_{t'=14}^{19}\binom{t'}{14} = \binom {20}{15}$$
And similarly the $\binom {16}{15}$ is the sum
$$\sum_{t=15}^{16}\binom{t-1}{14} = \sum_{t'=14}^{15}\binom{t'}{14} = \binom {16}{15}$$
And so
$$\begin{align*}
\Pr(X \le 16) &= \sum_{t=15}^{16} \Pr(X = t)\\
&= \sum_{t=15}^{16} \frac{\binom{t-1}{14}}{\binom {20}{15}}\\
&= \frac{\binom{16}{15}}{\binom {20}{15}}
\end{align*}$$
