A bacterium can divide every 20 minutes. Beginning with a single individual, how many bacteria will be there in the population if there is exponential growth for 3 hours:
P = C (I + r)n
Where, P = Final value
C Initial value
r = fraction increase
n = no. of times increase
In the given question, A bacterium can divide every 20 minutes and exponential growth is for 3 hours,
Thus, C = 1,
And r = 1 (it doubles every time)
n = 3 hours/20 minutes 9 times
Thus P = C (1 + = 1 (1 + 1)9 = 29 = 512
One Oilier Simple Way of Doing IT

Given information: A bacterium can divide every 20 minutes (3/hour)
Thus in 3 hours, it will total divide 3/hour x 3 hours = 9 times
Thus 9 times multiplication will successively yield 2, 4, 8, 16, 32, 64, 128, 256, 512 bacteria.
So at end of 9 hours, there will be 512 bacteria.
18
440
512
1024