## Elementary Algebra

When finding a common denominator, we may use any number that is a multiple of the denominators of all of the fractional addends. Since $20$, $40$, and $60$ are all multiples of both $4$ and $5$, any of them can be used as a common denominator. Since $20$ is the $\text{least}$ common multiple of $4$ and $5$, it is the $\text{least}$ common denominator.