2022 SSMO Accuracy Round Problems/Problem 10

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Problem

Let\[S=\sum^{\infty}_{k=5}2^{-10k}\dbinom{2k}{10}.\]Then the value of $S$ can be expressed as $\frac mn$, where $m$ and $n$ are relatively prime positive integers. Find the largest positive integer $a$ such that $2^a\mid mn$.

Solution