A grocer has 400 pounds of coffee in stock, 30 percent of which is decaffeinated. If the grocer buys another 100 pounds of coffee of which 60 percent is decaffeinated, what percent, by weight, of the grocer’s stock of coffee is decaffeinated?  Choose the correct choice from the following answers:
A) 28 %  B) 30 %  C) 36 %  D) 36 %  E) 40 %

Rationale: "1 . 30 % of 400 = 120 pounds of decaffeinated coffee 2 . 60 % of 100 = 60 pounds of decaffeinated coffee 3 . wt have 180 pounds of decaffeinated out of 500 pounds , that means 180 / 500 * 100 % = 36 % . the correct answer is d ."
Answer: D