Striver wrote:A certain candy manufacturer reduced the weight of Candy Bar M by 20 percent 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%

Say, initially the weight of each candy was W pounce and price of each candy was P.
Hence, initial price per ounce of each candy = P/W

After reducing the weight, weight of each candy bar = (W - 20% of W) = 0.8*W
After reducing the weight, price per ounce of each candy bar = P/0.8*W = 5P/4W

Hence, increase in price per ounce of each candy bar = (5P/4W - P/W) = P/4W
Hence, percentage increase in price per ounce of each candy bar = 100*(P/4W)/(P/W) = 100/4 = [spoiler]25%[/spoiler]

The correct answer is E.