There are $50$ bacteria at 10:00 and each divides simultaneously into 2 every 20 minutes. The process of dividing into two is known as binary fission. Let $n$ be the number of fissions that have occurred since 10:00. At what time does the tenth fission occur?
Expected answer is as follows.
The tenth fission occur $20 \times 10=200$ minutes after 10:00. As 200 minutes equal to 3 hours and 20 minutes, the tenth fission occurs at 13:20.
Unexpected answer is as follows.
As there are 50 bacteria that simultaneously divide themselves so all the first 50 fissions occur at 10:20. Thus the tenth fission, of course, occurs at 10:20 because all the 50 bacteria divide into two simultaneously at 10:20.
How should I reword the question to avoid the second answer above?
(I am not a native English speaker.)