GMAT PREP-2, Percent problem

[prasantnaidu]

A certain candy manufacturer reduced the weight of Candy Bar M by 20% but left the price unchanged. What was the resulting percent increase in the price per ounce of Candy Bar M ?

a.5%
b.10%
c.15%
d.20%
e.25%

The ans is E.
I am not able to determine what is the percent increase in the price when the price was unchanged. Any help would be great.
Thanks.

[yogami]

So let the weight of the candy be x units and the price be y
So its price per unit weight is y/x

New weight of the candy is 0.8 x but price remains unchanged i.e. y
So new price per unit is y/0.8x which is 5y/4x

So obviously you see 5y/4x is greater than y/x because 5/4 > 1
So the percentage of price per unit weight has increased.
The percentage increase is (5y/4x - y/x)*100/(y/x) = (5/4 - 1)*100 = 100/4 = 25%

[prasantnaidu]

Thanks yogami. it was a simple problem but i was confused in the words.

Keep Rocking,
Prasant.