confused - help please - exam on tuesday

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.

Thank you Stuart

This is a badly designed / worded problem .
What do you think ?
Thanx again

its from mgmat

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?

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

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