Mark gets picked up by the bus at 7: 40 he arrives at school at 8: 10 how long does it take to get to school

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
(7*40) + 40 = 460 minutes
490 - 460
= 30minutes