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You are told that V'(t) = 4 cm^{3}/sec and you want h'(t) at two different times. You can differentiate the expression above with respect to time but the right side will contain h'(t) and r'(t) which doesn't help since you don't have any information of r'(t). The key is the second diagram you sent. Triangles ABC and ADE are similar so 12/6 = (12  h)/r. Solve this equation for r and substitute into equation (1). This will give V(t) as a function of h alone. Differentiate this equation with respect to t. Can you complete the problem now? If not then write back and tell us what you have done and where you are stuck. Harley Luke wrote back
Hi again Luke, Your procedure is correct but you have an error in the derivative. It's
Harley  


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