A 79.0 kg person uses 248 kJ of energy to walk 1.00 km

Question

A 79.0 kg person uses 248 kJ of energy to walk 1.00 km. This energy comes from “burning” glucose (Burning 1.00 g of glucose produces 15.7 kJ of heat.), but only about 30.0% of the heat of combustion of glucose can be used for propulsion. The rest is used for other bodily functions or is "wasted" as heat. Assuming that a "sugar-coated" breakfast cereal contains 40.0% sugar (which can be considered glucose) and no other energy source, calculate how many grams of cereal provide enough energy to walk 1.00 km.

Answer 1

You can do a factor labelling analysis whenever you go from extensive variable to extensive variable (extensive variable is proportional to the amount of stufff - mass)

Start with what you know and cancel units to get to what you want:

1 km (kJ/km)(kJ from G/kJ needed)(g G/kJ from G)(g Cereal/g G) = grams Cereal

Note all per cents can be taken care of as they are only ratios.

248 kJ/ 1km, 100 kJ/30 kJ, 1g/15.7 J, 100 g C/40 g G