If a food label identifies the total number of kilocalories in a bowl of cereal as 140 and the carbohydrate kilocalories as 100, what is the approximate percentage of energy supplied by carbohydrates?

71.4%

This is a simple exercise in division. What percent of 140 is 100? A little simple division will provide the answer: 100 / 140 = 0.714285714 = 71.4285714%

Rounding to 3 significant figures gives 71.4%