The number of functions f from {1, 2, 3,..., 20} onto {1, 2, 3,..., 20} such that f(k) is a multiple of 3, whenever k is a multiple of 4, is:

The number of functions $\displaystyle f$ from {$\displaystyle 1$, $\displaystyle 2$, $\displaystyle 3$, ..., $\displaystyle 20$} onto {$\displaystyle 1$, $\displaystyle 2$, $\displaystyle 3$, ..., $\displaystyle 20$} such that $\displaystyle f(k)$ is a multiple of $\displaystyle 3$ , whenever $\displaystyle k$ is a multiple of $\displaystyle 4$, is :