[" The half-life of cobalt- "60" is "5.25" years.How long after a new sample is delivered,will the activity have "],[" decreased to about one third "(1/3)" of its original value."],[[" (1) "5.25" years "," (2) "8.3" years "," (3) "10.50" years "," (4) "15.75" years "]]