You are inflating a large spherical balloon at the rate of $17 cm^3 sec^{-1}$. How fast is its radius increasing when the radius is 20cm?
Given $17 cm^3 sec^{-1}$, shouldn't the answer be ${3}\sqrt{17} cm sec^{-1}$? But that is not one of the answers... How can I solve this?
Hint: You know $\dot{V} = dV/dt = 17 \text{cm}^3/\text{s}$. You want $v_R = \dot{R} = dR/dt$ at that point in time when $R(t) = 20 \text{cm}$.
I would start with relating volume and radius, then differentiate both sides regarding time.