# Math Help - Balloon's growth speed

1. ## Balloon's growth speed

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
$V(r) = 4/3pi*r^3$
I also assume the ball's growth speed can be calculated by using
$V'(r) = 4pi*r^2$ ?

2. The volume $V$ may also be written as a function of $t$. In that case, the rate of change becomes

$V'(t)=\frac{d}{dt}\left(\frac{4}{3}\pi r^3\right)=4\pi r^2\frac{dr}{dt}.$