using the formula for an exponential function:
y=b*a^x
Insert you values:
y=200*2^x
b=200 because there are 200 bacteria in the beginning
a=2 because 200%/100%=2. (200 bacause it doubles)
Insert 20 in x and solve for y:
y=200*2^13=819200
So after 12 hours there are 819200 bacteria.
I hope that helped.
