Since it is a frustum, extend the sides on down to make a cone, then treat it as such.

If we extend the sides down to the apex, it is at (0,-6).

That means the cone is 8 inches high overall.

By using similar triangles:

When r=5, then h=20/3.

We have to find dh/dt, when h=20/3.

Solving for dh/dt, we get

Now, the easy way I mentioned was by noting that

Where A(t) is the area of the water surface at some time t. In this case, we want to know the surface of the water area when r=5.

That is easy enough,

Now, all we do is

. Same as above only much quicker.