66 stir frys put you over the carbohydrate limit Fat = 10*5 + 66*4 =294 (under the fat limit) Carbohydrates = 10*35 + 66 * 40 = 3040 (over the carbohydrate limit) (one less ) carb (65 stir frys) = 10*35 + 65*40 = 2950 (below the limit) You have to stay below both limits that why 75 is not an answer when you get to 76 , it puts you over the fat limit too.
I don't understand your question. Where is the difficulty ? I do not know what "65 should be something like 10" has to do with it. Where does the 65 actually come from ? And it has no connection with the number of Szechuan chicken meals which were ordered. 65 is the correct answer. What is it that does not make sense ?
The 10 Szechuan chicken meals will provide 50 g of fat and 350 g of carbohydrate. Out of his "allowance" of 350 g of fat and 2975 g of carbohydrate, he can buy stir-fry meals providing up to 300 g of fat and 2625 g of carbohydrate. If he bought more than 75 stir-fry meals, that would provide too much fat. If he bought more than 65 stir-fry meals, that would provide too much carbohydrate. He CAN buy 65 stir-fry meals, which will provide 260g of fat and 2600g of carbohydrate. Adding that to the 50g of fat and 350g of carbohydrate that came from the Szechuan chicken, he will now have food providing 310 g of fat and 2950 g of carbohydrate. So the 65 meals is correct, and I have no idea where you got "10".