Need help with this task =/

Dad is blowing up a balloon for his son so that the volume of the balloon is growing 1 litre per second. How fast is the ball growing, when

a) the balloon's volume is 4 litres

b) the balloon's diameter is 30 cm ?

I know that I can calculate the volume of the balloon with

$\displaystyle V(r) = 4/3pi*r^3$

I also assume the ball's growth speed can be calculated by using

$\displaystyle V'(r) = 4pi*r^2$ ?