2h35min00sec-1h42min25sec=

2h 35min-1h=1h 35min

1h 35min-42min=53min

53min-25sec=52min 35sec

Answer is 52 minutes & 35 seconds.

2 hr 35 min 00 sec can be written as   1hr 95min = 1hr 94min 60sec

So

   1 hr 94 min  60 sec

-  1 hr 42 min  25 sec

---------------------------

           52 min  35 sec

Just as thejamesmachine found !!!!