Answer:
i wanna say 11 forgive me if wrong.
Explanation:
u=speed, w=wavelenght, f=frequency
It's known that u=w*f => f=u/w
u=20m/s ==> f=20/0,5 => f=40 Hz
w=0,50m
The answer is A because the equation is KEi+PEi=KEf+PEf
i means initial (before) and f means final (after)
