posted by amy .
An investigator receives Co-60 (5.27 year half life) for use in a research study.
Unfortunately the Co-60 is contaminated with Cs-137 (30.0 year half life). The
initial Co-60 activity is 400 times the initial Cs-137 activity. How long after the
initial assay will the Cs-137 activity be 0.2 times the Co-60 activity?