A conical reservoir has a depth of 24 feet and a circular top of radius 12 feet. It is filled so that the depth of water is increasing at a constant rate of 4 feet per hour. Determine the rate in cubic feet per hour at which the water is entering the reservoir when the depth is 5 feet.

I believe that a conical reservoir looks like this:

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0/000000\0

/00000000

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don't you have to know how to find the bottom radius in order to solve this problem (and i can't do that)?

thanks