The grams of carbohydrate in one apple if the apple has no fat and no protein and provides 72 kcal of energy

Carbohydrates have 4 kcal per gram of carbohydrates. (Protein is also 4 kcal/gram, while fat is 9 kcal/gram). Take the total kcal and divide by 4 kcal/gram: 72 kcal / (4 kcal/gram) = 18 grams carbohydrates.