The radius of a cone is increasing at a rate of 3 centimeters per second and the height of the cone is decreasing at a rate of 4 centimeters per second. At a certain instant, the radius is 8 centimeters and the height is 10 centimeters.
What is the rate of change of the volume of the cone at that instant (in cubic centimeters per second)?

the solution is h=10 because you add 9 to each side of the equation.

the revenue of the fitness center, y, is the product of two factors: the number of members and the fee paid by each member. there are 320 members now, but number of members goes down by 10 for each $5 increase in fees, so if x is the number of $5 price increases then members is equal to (320 - 10x). since the fee is currently $45, and the fee increases by $5 for every price bump x, the fee is given by (45 + 5x). so, revenue y is given by y = (320 - 10x)(45 + 5x).