I seriously have no idea where to start in these questions

tumor is a collection of cancerous cells. Suppose each cancerous cell has a radius of 5×10−3 cm, and that the doubling time for a population of cancerous cells is 20 hours.

If the tumor has an initial volume of 0.1 cm3, how many cancerous cells does it contain? (For simplicity, assume that the tumor is entirely made of cells, that there is no space between them.) Round your answer up to the next whole number.
find a formula for V(t), the total volume of the cancerous cells, after t hours. V(t)=
How many hours will it take before the volume of the tumor reaches 1 cm3? Round your answer up to the next whole number.

(0.1 cm^3)/(4π/3 * (5*10^-3 cm)^3)

= 0.1/(5.24*10^-7)
= 190986 cells

V(t) = 0.1 * 2^(t/20)