actually the info. that's given has no relation with the answer...........

Its just that in 60 minutes, the full cup is filled , It is a known fact that when a bacteria divides by binary fission.... it doubles itself in number. So after the cup is half full, only 1 more binary fission is required is required to fill the whole cup, as if each bacteria in the half filled cup is gonna undergo binary fission and double itself , then the whole cup would become full. Since 1 binary fission takes place in 1 minute.....we have to subtract 1 minute from the total time, i.e.60 - 1 = 59 minutes.

hope u understood wat i was tryin to convey!!!