USAMO 1988 Problem 1

The repeating decimal $0 . \mathrm{ab} \ldots \mathrm{kpq} \ldots \mathrm{u}=\mathrm{m} / \mathrm{n}$, where $\mathrm{m}$ and $\mathrm{n}$ are relatively prime integers, and there is at least one decimal before the repeating part. Show that $\mathrm{n}$ is divisible by 2 or 5 (or both). [For example, $0.011 \underline{36}=0.01136363636 \ldots=1 / 88$ and 88 is divisible by 2.]