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