Question

a population of insects doubles every month. this particular population started out with 20 insects

Answer 1

Umm youre missing some of his paragraph... How many months?

Answer 2

Start with 20
After 1 month, 20 * 2 = 20 * 2^1
After 2 months, (20 * 2) * 2 = 20 * 2^2
After 3 months, (20 * 2^2) * 2 = 20 * 2^3
After x months: 20 * 2^x