If a sample with 100% carbon-14 decays to a point where it has 50% carbon-14, how many half-lives have passed?

Bob, a nutritionist who works for the university medical center, has been asked to prepare special diets for two patients, susan and tom. bob has decided that susan's meals should contain at least 320 mg of calcium, 17 mg of iron, and 42 mg of vitamin c, whereas tom's meals should contain at least 280 mg of calcium, 13 mg of iron, and 34 mg of vitamin c. bob has also decided that the meals are to be prepared from three basic food