RS Aggarwal

Question

A clock is displaying correct time at 9am on Monday. If the clock loses 12 minutes in 24 hours, then the actual time when the clock indicates 8:30pm on Wednesday of the same week is

(a) 8 pm

(b) 7 pm

(c) 9 pm 

(d) 8: 59 : 45 pm

Answer 1

Take the option c)9 pm

It means from 9am on monday to 9pm on wednesday => 24 + 24 + 12 = 60 hours elapsed.

clock loses 12 minutes every 24 hours.i.e 0.5 minutes every hour

60 x 0.5 = 30 minutes.

9.00 pm - 30 minutes = 8:30 pm

Comments

From Monday 9 AM → Wednesday 8.30 PM = 59.5 hours

Clock loses 12 minutes in 24 hours,  hence loss per hour= 12/24 = 0.5 Minutes per hour

In 59.5 hours: 59.5 * 0.5 29.75 minutes = 29 minutes 45 seconds

The clock is 29 min 45 sec slow.So when it shows 8:30 PM, actual time is ahead by 29 min 45 sec.

So 8.30 + 29 minutes 45 seconds = 8:59:45 PM

Why not this then?

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8:30pm is the delayed time not the correct time.we need correct time and then apply loss to get the delayed time on the clock.

9am monday to 8:30pm wednesday is not true elapsed time but delayed time.

Answer 2

By intuitive approach

The answer is 9pm

It looses 12min every 24hr.

So from Monday 9am to Tuesday 9am, it looses 12min. The actual time shown on tuesday should be 9am, but it shows 8.48am. On wednesday, the actual time should be 9am, but it shows 8.36am.

At 8.30pm, the time duration is 24+24+12(at 8.30pm evening). = 60hrs.

This indicates 12+12+6 min lost; Which means 30mins.

The time showing is 8.30pm, BUT the time SHOULD be shown is 8.30+30=9pm
Therefore option c)