1 yuan a catty, that is, green onions and green onions are one yuan a catty. When he bought green onions and green onions respectively, the weight of green onions was 70 cents and that of green onions was 30 cents, but the unit price changed. Green onions are less than 3 cents a catty, green onions are less than 7 cents a catty, and the last 50 yuan was bought.

The simplest statement: according to the multiplication distribution rate, c(a+b)=ac+bc, in this problem, it is very easy to explain that the weight of green onions is substituted into A, the weight of green onions is substituted into B, and the money per catty is substituted into C.