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Twin Sisters A and B bought 2 wristwatches at 12 p.m .

An Hour later , A's watch reads 1:02 p.m while B's watch reads 12:56 p.m.

Later , on same day : If A's watch reads 10 p.m then at that time what is the reading on B's Watch ..?


a)exactly 9.00pm

b)approximately 9.00pm

c)exactly 9.20pm

d)approximately 9.20pm

e)exactly 9.40pm

f)approximately 9.40pm

g)None of these

Answer : NOT exactly known .

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up vote 1 down vote accepted

B’s watch covers $56$ minutes while A’s covers $62$, so B’s watch runs $\frac{56}{62}=\frac{28}{31}$ times as fast as A’s. At the time in question A’s watch has covered $10$ hours, or $600$ minutes, so B’s has covered


minutes, or a little under $9$ hours and $2$ minutes. The correct answer is therefore (b).

Added: Note that at $10$ p.m. A’s watch reads $10:20$, and B’s reads $9:20$. It was roughly $20$ minutes earlier when A’s watch read $10:00$ p.m., and at that point B’s also read roughly $20$ minutes earlier, or around $9:00$ p.m. This rough calculation already shows that the answer should be (a) or (b). And since A’s watch runs faster than B’s, backing up A’s watch by $20$ minutes to $10:00$ p.m. must back up B’s by less than $20$ minutes to something a little after $9:00$ p.m. This gets us the answer (b) without any messy arithmetic.

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Thanks.,.....:) – Jay Teli Feb 3 '13 at 23:57
You’re welcome. – Brian M. Scott Feb 4 '13 at 0:00

My Approach : I tried using methods that lead me to different answers :

One of which i fell MAY be correct is using the logic that there will be difference of multiples of 6 min( n hours => 6n min gap ) in the readings .

After 10 hours , gap would be 60 mins : A=10pm and B=9pm thus a is answer as per me ; some of my friends have got c , d .

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the twin sisters bought the watches but when did they set the time? so B's watch continues to show 6 minutes minus difference Time A =10:00 pm B = 9:54 PM

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answer is g. non of these – HS Bhatti Feb 19 at 12:07

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