If $\left(^{40}C_{0}\right) + \left(^{41}C_{1}\right) + \left(^{42}C_{2}\right) + \dots + \left(^{60}C_{20}\right) = \frac{m}{n} {}^{60}C_{20}$, $m$ and $n$ are coprime, then $m+n$ is equal to _____________.