The answer simple . The price of the potatoes is tripled ,so His consumption should be 1/3 times that of his normal consumption. E.g The rate is 20/kg . Then if he consumes 1kg per month he should consume only 335 grams approximately to spend the same money . The price of 1 kg is20 .And when tripled 60.1/3 of 60is 20 and so Id the quantity.