how much time does a boy spend in his school when he reaches at 9.50am and leaves from his school at 2.40pm?

Answer 1

we can rewrite 2:40 as 14:40
then we subtract,
14:40 - 9:50
since 40 is less than 50, we add an hour to it or 60 minutes, while we subtract 1 from 14:
13:100 - 9:50
4 : 50 = 4 hours and 50 minutes

hope this helps~ :)

Answer 2

4 hours and 50 minutes