Cassandra sets her watch to the correct time at noon. At the actual ti

We see that Cassandra’s watch loses 2 minutes and 24 seconds each hour or 144 seconds per hour. So if the correct time is 10 PM, her watch will lose 1440 seconds or 1440/60 = 24 minutes and the time on her watch would read 9:36 PM. Therefore, when her watch reads 10:00 PM (“24 minutes” later), the correct time should be slightly over 10:24 PM. Of the answer choices, we can eliminate A and B. Let’s guess that correct answer should be 10:25 PM, that is, for the correct time to elapse 25 minutes, the time on her watch only elapses 24 minutes. In other words, her watch loses 1 minute for every 25 minutes. Let’s verify that is indeed the case. We know that her watch losses 144 seconds every hour (i.e., every 60 minutes). So we can set up the proportion:

144 sec/60 min = x sec/25 min

60x = 144(25)

60x = 3600

x = 60

We see that her watch indeed losses 60 seconds (i.e., 1 minute) for every 25 minutes. So when her watch reads 10:00 PM, the correct time should be 10:25 PM.

Answer: C

total time in watch by 1 PM = 12 :57: 36 sec so the watch has done = 57*60 + 36 = 3456 seconds
watch is lagging every hour by 3600-3456 = 144 seconds
so in 10 hrs watch will lag by 144* 10 = 1440 seconds

now by 10 PM total seconds moved = 10*60*60 = 36000 seconds
so the actual time in watch must be by 36000/1440 = 25 ahead ; i.e 10:25
IMO C

the clock loses 2 minutes and 24 seconds every 60 minutes.

2.4/60=24/600=1/25. The clock loses 1 minute every 25 minutes. So when the correct clock goes 25 minutes the bad clock goes 24 minutes. We can think of it this way. The real clock GAINS a minute every time the bad clock goes 24 minutes.
Since the bad clock went for 600 minutes (10:00 pm is 10 hours after noon) we can create this proportion 1/24=x/600.
Solving for x we obtain 25 minutes. It must have gained 25 minutes. So 10:00+25 is 10:25.

Bunuel

should the seconds by which her watch is slow be divided by 60 so as to get the correct answer?
Her watch is slow by a total of 1440 seconds in 10 hours so that means if her watch shows 10 PM then the actual time has to be ahead by 1440 seconds right?

Pls correct me if im wrong

Exactly my thoughts, if by every hour the watch lags by 144 seconds, at the end of 10 hours shouldn't it have lagged by 1440 seconds which equals 24 minutes?
Thus the time shown should be 10:24 p.m?

let t=actual time
t/10*3600 sec=10/10*3456 sec
t=10.41 hours=10:25 PM
C

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.