In a certain colony of cancerous cells,each cell breaks into 2 new cells every hour.how many cells will the colony have at the end of the 9 hours?it is known that life of a cell is 20 hours?i)2^9-1 ii)2^9 iii)2^10 iv)2^10-1

Although in the question it hasn't been specified how many cells to start with. I am assuming it to be 1 considering the options.
It is essentially a GP with a=1, r=2 and n=9. So, the number of cells by the end of 9 hours will be-
[a(r^n-1)]/(r-1) which will give us (i) as the answer.