5. Define the operation $a * b=a b-5\left[\frac{a b}{5}\right]$, where $[x]$ denotes the greatest integer not exceeding the real number $x$, and the set $A=$ $\{0,1,2,3,4\}$. A bijection $f: A \rightarrow A$ satisfies $f(a * b)=$ $f(a) * f(b)$. Then the number of such $f$ is .