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GMAT prep - Percentage Problem

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sachinkr: Junior | Next Rank: 30 Posts; Posts: 28; Joined: Sun Nov 02, 2008 12:35 pm; Thanked: 7 times

GMAT prep - Percentage Problem

by sachinkr » Sat Jan 10, 2009 10:30 am

GMAT prep percentage problem

I got answer C. but answer is A. I tried statement A and assuming price of third item 20 and I get discount is more than 15% and when i take the price 10, i get discount less than 15%, so i chose C. I did not understand why A is sufficient below.

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Source: Beat The GMAT — Data Sufficiency | Sat Jan 10, 2009 10:30 am

Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pm; Location: Toronto; Thanked: 1710 times; Followed by:614 members; GMAT Score:800

Re: GMAT prep - Percentage Problem

by Stuart@KaplanGMAT » Sat Jan 10, 2009 10:55 am

sachinkr wrote:GMAT prep percentage problem

I got answer C. but answer is A. I tried statement A and assuming price of third item 20 and I get discount is more than 15% and when i take the price 10, i get discount less than 15%, so i chose C. I did not understand why A is sufficient below.

Let's do the math for (1).

If prices are 50, 20 and 20, then our discount is 10+2+2 = 14.

14/90 IS more than 15%, so we can get a "yes" answer.

If prices are 50, 20 and 0 (or so little above 0 so that it doesn't matter), then our discout is 10+2+0 = 12/70.

12/70 IS more than 15%, so we get another "yes" answer.

In fact, the lower you make the cost of the 3rd item, the higher our % discount gets, since then the 20% on the highest item will have more weight in the overall average discount. So, as soon as we see that the 50/20/20 scenario gives us a total discount over 15%, we can reason that the total discount will always be bigger than that.