Surface area and volume — practice question

The volume of a sphere is $\frac{4}{3}\pi r^3$. In a storm, raindrops collect inside a cylinder that is standing on level ground. Before the storm began, the cylinder contained no water. The cylinder has radius $20\text{ mm}$. Each raindrop is a sphere with radius $2\text{ mm}$. When the storm has ended, the water depth in the cylinder is $16\text{ mm}$.