24 hour conundrum
It is 225 minutes until midnight. What time is it on a 24-hour digital clock?
Age
11 to 14
Challenge level
Problem
It is 225 minutes until midnight. What time is it on a 24-hour digital clock?
This problem is taken from the UKMT Mathematical Challenges.
Student Solutions
Answer: 20:15
1 hour = 60 minutes
2 hours = 120 minutes
3 hours = 180 minutes
4 hours = 240 minutes
Using 3 hours = 180 minutes
180 + 45 = 225, so 225 minutes = 3 hours and 45 minutes.
3 hours before midnight is 21:00
45 minutes before 21:00 is 20:15
Using 4 hours = 240 minutes
240 $-$ 15 = 225, so 225 minutes is 15 minutes less than 4 hours.
4 hours before midnight is 20:00
15 minutes after 20:00 is 20:15