Knowing the amount doubles every 30 days, we'll have to find how many times it'lll double in the 210-day time frame:
210 / 30 = 7
It doubles 7 times. If we were to write a rough, not worked on expresion representing that, knowing the starting population was of 20, we would write:
(((((((20 * 2) * 2) * 2) * 2) * 2) * 2) * 2) = 2560 rabbits
Hope it helped,
Happy homework/ study exam!