Example 4: Real-World Problem Solving p440
Medicine The half-life of a radioactive substance is the length of time it takes for
one half of the substance to decay into another substance. To treat some forms of
cancer, doctors use radioactive iodine. The half-life of iodine-131 is 8 days. A
patient receives a 12-mCi (millicuries, a measure of radiation) treatment. How
much iodine-131 is left in the patient 16 days later?
In 16 days, there are two 8-day half-lives.
After one half-life, there are 6 mCi left in the patient.
After two half-lives, there are 3 mCi left in the patient.
a. How many half-lives of iodine-131 occur in 32 days?
b. Suppose you start with a 50-mCi sample of iodine-131. How much iodine-131 is
left after one half-life? After two half-lives?
c. Chemistry Cesium-137 has a half-life of 30 years. Suppose a lab stored a 30-mCi
sample in 1973. How much of the sample will be left in 2003? In 2063?