Let's sum these two numbers.
8 + 21 = 29 total hours.
In every 12-hour the clock format changes AM to PM or the opposite PM to AM. So we need to divide 12 with the result 29.
There are 2 times 12 number in the 29
If the result is ODD number the time format will change the opposite side. If the result is EVEN number the time format will be the same.
Answer :5pm
What is 21 hours from 8pm ? = 5pm.
Add 21 hours to 8pm is equal 5pm.