Sorry, this "9/32pi m/min" is velocity? Level of velocity?
Write r in terms of h.
If you split the cone vertically, you have a right-angled triangle,
height H=15 and width R=4.
As the water flows in, the height and radius of the water vary, but they
still have the same ratio
As the water enters at per minute
Now substitute your value for
to find "h squared" and take the square root.