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 ]
