Answer:
9 days
Step-by-step explanation:
He sets his watch 12 seconds behind
For every day that passes it falls back another 1 second
So once it is 21 seconds behind all you have to do is subtraction to find the amount of days past
21 - 12 = 9
Since its 12 seconds behind already itll take 9 days to be behind 9 more seconds.
