The price of lunch for 15 people was $207.00 including a 15 % gratuity for service . What was the average price per person , Excluding the gratuity ?
A. $11.73
B. $12.00
C. $13.80
D. $14.00
E. $15.87
[spoiler]Ans = B [/spoiler]
If you consider 207.00 as 115% and solve then you get the correct answer . But if you take 15% of 207 and subtract that from 207 and divide the result by 15 you will get the answer as 11.73 which is not correct .
My question is why is the second approach not correct , when there should not be any difference in answers no matter what the approach . If we look at the two approaches we are basically doing the same thing so why different answers ?
Can any one please explain ?
OG #12 PS #123
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Hi rockeyb,
This is a basic concept of percentages.
Say, if you add x% of a certain value onto that value to get y. Taking x% of y off from y will not get you back to the original value. Try it with an example.
Say 20% of 100. If you add 20% to 100 you get 120. On the other hand, 20% of 120 is 24. If you take away 20% from 120, you get 96 (and not 100!)
This is simply because 20% of 120 is larger than 20% of 100.
This is a basic concept of percentages.
Say, if you add x% of a certain value onto that value to get y. Taking x% of y off from y will not get you back to the original value. Try it with an example.
Say 20% of 100. If you add 20% to 100 you get 120. On the other hand, 20% of 120 is 24. If you take away 20% from 120, you get 96 (and not 100!)
This is simply because 20% of 120 is larger than 20% of 100.
- rockeyb
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Thanks for your reply valleeny ,valleeny wrote:Hi rockeyb,
This is a basic concept of percentages.
Say, if you add x% of a certain value onto that value to get y. Taking x% of y off from y will not get you back to the original value. Try it with an example.
Say 20% of 100. If you add 20% to 100 you get 120. On the other hand, 20% of 120 is 24. If you take away 20% from 120, you get 96 (and not 100!)
This is simply because 20% of 120 is larger than 20% of 100.
I got it , the reason the second approach dose not work is because 207 is 115% and not 100% so taking away 15% of 207 will not give me 100% but some thing less than 100% .
And answer option A is exactly the kind of trap answer that GMAT throws at you , well a lesson learnt . Thanks for explaining.
- Lattefah84
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thank you both for revealing this trap, I also got 11.73