Question 873432
You start out with some amount of bacteria (Q0)



It doubles after 9 hours (Q0 ----> 2*Q0)



After another 9 hours (now at 9+9 = 18 hours), it doubles again: 2*Q0 ----> 4*Q0



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We've gone from Q0 to 4*Q0



Divide to get (Q0)/(4*Q0) = 1/4



So 1/4 of the population of bacteria after 18 hours is the original amount you started out with.