6

6. [Maximum mark: 6]

The volume of a spherical bubble increases at a constant rate of $5 \text{ cm}^3 \text{s}^{-1}$.
The initial volume of the bubble can be assumed to be zero.
Find the rate, in $\text{cm s}^{-1}$, at which the radius of the bubble is increasing when the volume of the bubble is $20 \text{ cm}^3$.