calculate the elapsed time between 2h 42min 23sec and 4h 38min 12sec

      4   38  12

 -    2  42  23

=    1  55  49

We can do this by "borrowing"   ......just like we often do in subtraction problems.....

Note   

4h 38min 12sec   [borrowing seconds from a minute  ]   = 

4h 37 min + 60  sec + 12 sec  =

4h 37 min 72 sec   [ now.....borrow  minutes from an hour]  = 

3 h + 60 min + 37 min + 72 sec  =

3h 97 min 72 sec

So

  3h 97 min 72 sec

- 2h 42min  23sec   =

  [3h - 2h] + [97 min - 42 min ] +  [72 sec - 23 sec]  =

       1h                55m                       49sec                        [as EP found ]