The above answer is the easiest, but if you forget, I always suggest to look at the units.
In this case we have m^3/s.
You are given velocity, so it makes sense to have the answer equal velocity x something.
Then, you are given the diameter of a supposedly round object. What can you determine with that?
The area, which is pi*r^2 = pi*(d/2)^2 = pi/4 * d^2 with units of m^2.
These multiply to give you the answer, but can be determined without knowing a formula.
Applicable to any unit-based question.