10 minutes before 9 is 8:50. since any time on the hour is at a perfect 60 minutes, you subtract the minutes given and move 1 hour back to find your answer. for example, if you were asked what 20 minutes before 10 was. you start by subtracting 20 from 60. 60-20= 40, this being your minutes. 10-1 is 9, this being your hour, making the final answer 9:40.