計算機數學

The following questions constitute a set. Please answer accordingly.

Let $X = \{1,2,3,4,5,6\}$ and $Y = \{a,b,c,d\}$. For a function $f$ and a set $S$, define $f(S) = \{f(i) \mid i \in S\}$. Please find the answers for the following questions.

(4分) How many functions $f: X \to Y$ are onto (surjective)?