A couple who own an appliance store discover that if they advertise a sales discount of 10% on every item in the store, at the end of one month the number of total items sold increases 15%. Their gross income from sales for one month increases by what percent? Choose the correct option
A) 1.5 % B) 2.5 % C) 3.5 % D) 4.5 % E) 5.5 %

Rationale: "let p be the original price and let x be the number of items sold originally . the original income is p * x . after the changes , the income is 0.9 p * 1.15 x = 1.035 * ( p * x ) , an increase of 3.5 % . the answer is c ."
Answer: C