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confused - help please - exam on tuesday

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confused - help please - exam on tuesday

by hpgmat » Sun Dec 13, 2009 2:50 pm
Cindy has her eye on a sundress but thinks it is too expensive. It goes on sale for 15% less than the original price. Before Cindy can buy the dress, however, the store raises the new price by 25%. If the dress cost $68 after it went on sale for 15% off, what is the difference between the original price and the final price?

$0.00
$1.00
$3.40
$5.00
$6.80


the answer that i get is not in the choices given.

What am i doing wrong? is the question correct ?

0.85x = 68
x= 80

1.25 (X) - X= 20
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by Stuart@KaplanGMAT » Sun Dec 13, 2009 3:43 pm
hpgmat wrote:Cindy has her eye on a sundress but thinks it is too expensive. It goes on sale for 15% less than the original price. Before Cindy can buy the dress, however, the store raises the new price by 25%. If the dress cost $68 after it went on sale for 15% off, what is the difference between the original price and the final price?

$0.00
$1.00
$3.40
$5.00
$6.80


the answer that i get is not in the choices given.

What am i doing wrong? is the question correct ?

0.85x = 68
x= 80

1.25 (X) - X= 20
Here's the problem:
the store raises the new price by 25%
The 25% increase is on the sale price, not the original price.

So, it's:

1.25(sale price) - (original price) to get the answer,

or:

1.25 (68) - 80 = 85 - 80 = 5... choose D.
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by hpgmat » Sun Dec 13, 2009 4:02 pm
Thank you Stuart

This is a badly designed / worded problem .
What do you think ?
Thanx again
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by Ian Stewart » Sun Dec 13, 2009 4:15 pm
The wording is far too casual, and most of the details in the question are irrelevant (why mention Cindy at all?); it is not at all like a real GMAT question in that sense. Where is the question from?
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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by hpgmat » Mon Dec 14, 2009 6:18 pm
its from mgmat
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by Gurpinder » Tue Dec 15, 2009 10:01 am
hmm sorry if i am wrong but shouldn't the answer be 6.80

68 (sale price at 15%). so 68x1.15=78.2 << the original price

and the 25% ^ is on the sale price. so 68x1.25=85

now 85-78.2 = 6.8

am i doing something wrong?
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by Stuart@KaplanGMAT » Tue Dec 15, 2009 11:18 am
Gurpinder wrote:hmm sorry if i am wrong but shouldn't the answer be 6.80

68 (sale price at 15%). so 68x1.15=78.2 << the original price

and the 25% ^ is on the sale price. so 68x1.25=85

now 85-78.2 = 6.8

am i doing something wrong?
Arrrgggg.. I've typed out a reply twice so far and the website has eaten it. My last attempt!

You've made a very common math error when dealing with percent increases and decreases.

You've turned a 15% decrease the original amount into a 15% increase of the new amount, which is a misapplication of the percent formula.

One way that we can express the percent formula is:

% * whole = part

A 15% decrease is the same as "85% of", so:

85% * whole = part

swapping to fractions:

17/20 (whole) = part

and multiplying both sides by 20/17 we get:

whole = (20/17) part

20/17 is NOT 1.15, which is why your math doesn't work.

Applying the above to our part of 68, we get:

whole = 20/17 * 68/1

whole = (20*68)/17 = 20 * 4 = 80
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by Gurpinder » Tue Dec 15, 2009 12:25 pm
ahhh

i get it. I was making the mistake of 68x1.15 < rather I should do it like 68+(68x .25) = 85

thank you very much
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