Bob atarted mowing at 9: 55am it took him 25 minutes to mow the front yard and 45 minutes to mow the back yard. at what time did bob finish mowing?

So bob started at 9:55. you add the 25 minutes from how it took him to mow the front yard to your starting time

9:55    there are sixty minutes in an hour
+:25
10:20 am 
 then you add the 45 minutes it took him to mow the back yard from that time

10:20     remember there are only sixty minutes in an hour
+  :45
11:05 am 
 
so Bob finished mowing at 11:05am