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GMAT Paper Test (Code 25, Q#11) - need clarification

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by Nisha1218 » Fri Sep 07, 2007 5:58 am
If 1/2 of the air in a tank is removed with each stroke of a vacuum pump, what fraction of the original amount of air has been removed after 4 strokes?

A. 15/16
B. 7/8
C. 1/4
D. 1/8
E. 1/16

The OA is A. 15/16

I got the answer by assuming that "32" is the capacity of the tank, so after
Stroke #1 16 air is remaining
Stroke #2 8 air is remaining
Stroke #3 4 air is remaining
Stroke #4 2 air is remaining
Total Air Removed 30

30/32 = 15/16

However, if you assume that "16" is the capacity of the tank, then you arrive to the answer of 7/8 (choice B)

Stroke #1 8 air is remaining
Stroke #2 4 air is remaining
Stroke #3 2 air is remaining
Stroke #4 0 air is remaining
Total Air Removed 14

14/16 = 7/8

So is there any way to solve the above question so you always arrive to the OA of 15/16????
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by samirpandeyit62 » Fri Sep 07, 2007 6:20 am
in the second explanation half of 2 is 1 and not 0

I think you overlooked that

if you take it 1 you will get the correct ans 15/16

I would like to add one more thing, when picking up numbers always try to pick up smart nos i.e nos which will help you to perform the required operation in the question easily.

e.g here we know that we would require to cut the nos in half 4 times hence choose a nos which can be easily cut into half 4 times like 16 etc and not a nos like 8.

Regards

Samir
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by Nisha1218 » Fri Sep 07, 2007 8:16 am
duh...what a careless mistake...thanks samir!
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