# Repunit Factors

**Number Theory**Level 5

A repunit is an integer that consists of only copies of the digit \(1\). Let \(R_n\) be a repunit with \(n\) digits. Let a *ministring* be a sequence of digits composed of a nonnegative integer amount of the ' \(0\)' digit followed by a positive integer number of the ' \(1\)' digit. Find the number of divisors of \(R_{20}\) that consist entirely of copies of a single *ministring*.