OG #12 PS #123

Expert replies

rockeyb: Legendary Member; Posts: 537; Joined: Sun Jul 19, 2009 7:15 amFollowed by: 1 member; Location: Nagpur , India; Thanked: 41 times

by rockeyb » Sat Dec 26, 2009 10:43 pm

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 ?

Source: Beat The GMAT — Problem Solving | Sat Dec 26, 2009 10:43 pm

valleeny: Senior | Next Rank: 100 Posts; Posts: 59; Joined: Mon Nov 16, 2009 4:54 pmFollowed by: 1 member; Thanked: 8 times

by valleeny » Sat Dec 26, 2009 11:18 pm

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.

rockeyb: Legendary Member; Posts: 537; Joined: Sun Jul 19, 2009 7:15 amFollowed by: 1 member; Location: Nagpur , India; Thanked: 41 times

by rockeyb » Sun Dec 27, 2009 1:59 am

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.

Thanks for your reply valleeny ,

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: Senior | Next Rank: 100 Posts; Posts: 93; Joined: Mon Dec 14, 2009 4:15 am

by Lattefah84 » Sun Dec 27, 2009 3:04 am

thank you both for revealing this trap, I also got 11.73

https://photos-photos-of-nature.blocked/

Everything is possible!!!

Featured schoolRound 2 closes Jan 7, 2027 54 days left

Chicago Booth

Since 1982, 10 Booth faculty members have received the Nobel Prize in Economic Sciences, more than any other business school.

#3
U.S. News rank: $175,000
Median base salary: 670
Average GMAT Focus: 635
Students in the class

Apply to Booth Visit chicagobooth.edu See the full profile

Photo: Warren LeMay, CC BY-SA 2.0

GMAT Course Reviews

563 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (563 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review