Explanation:
you do know that a day has
24 hours = 24×60 = 1440 minutes
as every hour has 60 minutes.
starting at 10:09 am 24 hours would mean 10:09 am on the next day.
but it ended already at 9:30 am.
so, 30 + 9 = 39 minutes are missing to have the full 24 hours.
as 60 - 39 = 21 minutes, the elapsed time was
23 hours and 21 minutes = 1440 - 39 = 1401 minutes.
FYI : we could also simply count forward :
10:09 am to 11:00 am are 51 minutes.
11:00 am to 12 pm is 1 hour.
12pm to 12 am are 12 hours.
12 am to 9:00 am are 9 hours.
9:00am to 9:30 am are 30 minutes.
the sum is then
1+12+9 = 22 hours and 51+30 = 81 minutes.
81 minutes is 1 hour and 21 minutes.
so, the total sum is
22+1 = 23 hours and 21 minutes.
tada !