6. Cobalt-60 is used extensively in medicine as a source of gamma ray. Its half life is 5.27 years. a. how long will it take for cobalt-60 source to decrease to 18% of its original activity? b. What percent of activity remains after 29 months?

4. The half-life of radioactive radium-224 is 3.50 days. What was the original mass if 0.200 g remains after 10.50 days? A. 0.025 g B. 0.050 g C. 1.60 g D. 0.60 g

Radioactive vanadium-48 decays with a half-life of 16.0 days. a. What is the value of k in s? k = b. A sample contains 84.0 mg *°V. How many decay events will be produced in the first second? Assume the atomic mass of **V is 48.0 u. decays/s c. A chemist obtains a fresh sample of 48 V and measures its radioactivity. She then determines that to do an experiment, the radioactivity cannot fall below 25% of the initial measured value. How long does she have to do the experiment? days