The Consumer Goods Price Index is calculated on the basis of price developments for three categories of consumer goods: A, B and C, which account for 25%, 55% and 20% respectively of total consumer expenditure. Knowing that compared to the previous year, the price of good A increased by 5%, the price of good B increased by 10% and the price of good C increased by 7%, determine the inflation rate: a) 8.05% b) 7.65% c) 10% d) 8.15%

Why is d) the correct answer? How do we calculate?