If it is 5 o'clock, what time was it 28
hours ago? Or if I ask you what time it will be in 35 hours, can you tell me
the answer immediately? Of course, you can. We can calculate the answer to this
using a mathematical trick.
Arithmetic modulo 12 is called Clock arithmetic because the
remaining numbers modulo 12 are 0,1,2,3...11. These are the numbers on a clock
face, where 0 represents 12. Using this, all questions related to Clock
arithmetic can be answered.
1. What time is it 18 hours after 9 o'clock on a 12-hour clock?
Clock = 12 hour clock
Present time = 9’o clock
After time = 18 hours
9+18=27
12)27(2
24
------------
3
-------------
The answer is 3’o clock
2. What time is it 71 hours before 9 o'clock on a 12-hour clock?
Clock = 12 hour clock
Present time = 9’o clock
Before time =71 hours
9-71 = -62
12)62(5
60
------------
2
-------------
r’ = 2
r = m-r’
r = 12-2 = 10
The answer is 10’ o clock
Read more: Are AI smartwatches powered by artificial intelligence?