15) D

Let price of 1 biscuit = P

So price of 3 biscuits should be = 3P

Price of 4 biscuits = 4P

Since the shop has an offer price of 4 biscuits = 3P

According to the offer, price of 1 biscuit = (3P ÷ 4) = 0.75P

Decrease in price = P – 0.75P = 0.25P

Percentage decrease in price = (0.25P ÷ P) x 100 = 25%

Since price decreased by 25%, profit will also be decreased by 25%.

Let original profit = M

Profit after offer = M – 0.25M = 0.75M

Since the shop is still making a profit of 20%,

0.75M = 120%

M = (120 ÷ 0.75) = 160%

So profit = 60%