A certain bacteria colony doubles in size every day for 21 days, at which point it reaches the limit of its habitat and can no longer grow. If two bacteria colonies start growing simultaneously, how many days will it take them to reach the habitat’s limit? Choose the correct choice from the following answers:
A) 6.33
B) 7.5
C) 20
D) 15
E) 19

Rationale: "if there is one bacteria colony , then it will reach the limit of its habitat in 21 days . if there are two bacteria colonies , then in order to reach the limit of habitat they would need to double one time less than in case with one colony . thus colonies need to double 20 times . answer : c . similar questions to practice : hope it helps ."
Answer: C