She was on the bus for :20 before 3:00 and for :15 after 3:00, for a total of ...
:20 +:15 = :35
That is, 35 minutes.
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There are 60 minutes in an hour. If you like, you can change an hour to 60 minutes and write the exit time as 3:15 = 2:75. Then you can perform the subtraction
