cleetus wrote:

What if the question asks u to find only the time difference between two countries and no information about the speed, i.e, 525 km per hour is not mentioned in the question. Do u need to do the lengthy algebraic calculation for 3600/(s+75) + 3600/(s - 75) = 14 . Or is there any easy Go.

You will almost never need to solve this equation and in GMAT, I can say will reasonable certainty that you will 'never ever' need to solve it the long way. In such questions time taken is generally an integral value.

Even if 525 is not there in the options, look at the big picture

3600/(s+75) + 3600/(s - 75) = 14

The two terms on the left hand side add up to give 14 hrs. Obviously the first term is smaller and the second term is greater. I can split 14 into 6 hrs and 8 hrs OR 5 and 9 OR 4 and 10 etc

Now, wind will have some effect on the time taken but it may not be very dramatic. So, I will first try 6 and 8.

I say, what if 3600/(s + 75) is 6 hrs? i.e. what if s+75 is 600? i.e. what if s is 525? I plug s = 525 in 3600/(s - 75) and see that I do get 8. Then s must be 525!

Had 6 and 8 not worked, I would have tried 5 and 9.

Another thing:

Say the equation is: 770/(s + 20) + 770/(s - 20) = 18

I can split 18 into 8 and 10 OR 7 and 11 etc.

I will not try 8 and 10 here because 770 is not divisible by 8 so no way can I get an integer value for the time i.e. I cannot get 8 hrs.

I will jump to 7 and 11 first.

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