# A round balloon is being blown up with helium so that t seconds after starting, the radius is 2t cm. Remember that for a sphere with a radius r, V=(4/3)(pi)r^3 and SA= 4(pi)r^2. What is the equation for V'(r)?

## V=(4/3)(pi)r^3

Just to be clear, you used the prime notation, $V ' \left(r\right)$ (which is also written as $\frac{\mathrm{dV}}{\mathrm{dr}}$).
So here: $V ' \left(r\right) = {\left(\frac{4}{3} \pi {r}^{3}\right)}^{'} = 4 \pi {r}^{2}$