after 20 hours the clock will show 3*20 = 60 min = 1 hr more
since the clock is analogue after 12 hours it will show the same time hat is 6 am and 6 pm are shown the same way.
after 20*12 = 240 hr = 10 days the clock will show 12 hr more but since 6 am and 6 pm are shown the same after 240 hr = 10 days the clock will show the correct time again.
the correct time will be shown on Monday 6 pm.