Do this in chunks. 10:45 up to 11 o'clock is 15 minutes. 11am to 1pm is 2 hours. 1 to 1:05 is 5 minutes. Add the 5 minutes to the 15 to get 20 minutes then put in the hours: 2 hrs 20 mins. It helps to visualise or use a clock face, because you know the major divisions of the hour are in 5 minute segments: 5, 10, 15, 20, ... .