This can be calculated using the formula $nCx = \frac{n!}{x!(n-x)!}$, where $n!$ is the factorial of $n$, $x!$ is the factorial of $x$, and $(n-x)!$ is the factorial of $(n-x)$. In this case, $n=20$ and $x=17$, so we have $nCx = \frac{20!}{17!(20-17)!}$.
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