Question

a watch which loses uniformly was observed to be 12 minutes fast at 4:00am on 6th of a month. it showed 20minutes less than the correct time at 6:00pm on the 10thof the same month. when did the watch show correct time???? this is mental ability question

Answer 1

at 4 am it is 12 min fast.

at 6pm it is 20 min slow.

it means 12+20 min reduced in (6 pm on 6th-4 am on 10th) hrs i.e 110 hrs

finally in 110 hrs it is decreased 32 mins. 

110 hrs-- 32 hr
??? hrs- 12 hr it gives 110*12/32

it means
41.25 hrs+ 4 am on 6th

i.e. 1 day + 17 hours 15 mins + 4:00 am on 6th

i.e. 17 hrs 15 mins + 4:00 am on 7th

i.e. 9:15 pm on 7th which is the correct answer

Answer 2

Duration between the two observation points of time:  110 hours
  4 AM  on 6th  to   6 PM on 10th

Loss of time = 12+20 = 32 min

Rate of loss: 32 / 110    min per hour
Correct time is shown exactly after :   12 / 32  * 110
            = 41.25 hrs = 41 hrs 15 min.

So exactly at :  7th night at   21 hrs 15 min