you could do this in two ways, you could either subtract immediately or convert the time given in hours into minutes
STEP 1:
8:10
- 7:40
which will give 0:30
STEP 2:
8 hours 10 minutes into minutes is
(8 60) +10 = 490 minutes
7 hours 40 minutes into minutes is
(740) + 40 = 460 minutes
490 - 460
= 30minutes
