A jar contains a bacterium which doubles every minuteThe jar was full at the end of 3 hoursHow long will it take ot filll 25 percent of the jar?

That depends a great deal on how you define the term "multiply". If you mean each cell divides - doubling the number of bacteria, the it would be half full at 11:59 PM and double to twice that 1 minute later at 12 AM, i.e. 2 x ½ = 1 (12 AM is midnight - see the discussion below) . Of course if by 12 AM you meant Noon - then the answer would be 11:59 AM.

If the multiplication factor is other than 2, the answer would have to be modified accordingly.

Convention has it that 12 midnight is called 12 AM, which comes after 11:59:59 pm, and thus the day starts with am. In other Words the date in the calendar change at midnight and that moment is called 12 AM.

You could, of course , use a different system to avoid this confusion such as the 24-hour clock. There, the beginning of the day (12 AM or 12 midnight) is denoted by 0000 hours, and the last minute of the day starts at 2359 hours (equivalent to 11:59 pm). So the time after noon is denoted by numbers from which 1200 should be subtracted to get the time as per 12-hour clock.