In problems that start off taking a percent or fraction of an unknown you should fill in numbers for that unknown. In this problem the unknown is the original weight can cost of the candy bar.
Pick 100 ounces for the candy bar and 400 cents for the price.

Becuase the question asks for price increase per ounce we have to figure out what the original price per ounce is. That is found by 400/100 or 4 cents per ounce.

Then when the weight is reduced, the new weight is 80 ounces and the price stays at 500. so we need to get price per ounce by 400/80 or 5 cents per ounce. The increase is price was 1cent per ounce

Now we need to get the price increase so we use the question 1 cents is what percent of 4 cents.

that translates to 1 = x/100(4) - which solves to 25 so the increase in price was 25%.

The correct answer is E.

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

A) 5%

B) 10%

C) 15%

D) 20%

E) 25%
can anyone plz explain me in clear approach

say price = $100, and weight = 100 ounces

price/ounce= 100/100 = $1/ounce

weight dropped by 20%

so new weight = 100 * 0.8 = 80 ounces

or

100 - (100 * 20%) = 100 - 20 = 80 ounces

price/ ounce now = 100/80= $ 1.25

% increase = 1.25 - 1/1 = 0.25 or 25%