Add the minutes: 45+25=70, but 70 minutes is 1 hour and 10 minutes, so we add 1 to the hours to make 11 and the minutes = 10, so the finish time is 11:10.
Another way is to use a clock face, and in your mind's eye visualise the minute hand moving around. 15 minutes is quarter of an hour, which is a right angle on the clock. 45 minutes is three quarters. So move the minute hand through 3 right angles: 25 goes to 40 goes to 55 (diameter) goes to 10 minutes after the next hour.
Another way: 45 minutes is 15 minutes less than an hour, so we add an hour and take away 15 minutes: 11:25 take away 15=11:10.