Since we need to find the number of half-lives in 26.3 years, we simply divide 26.3 yrs by 5.26 yrs which yields 5 half-lives. This means that in 26.3 yrs the 200 grams of cobalt-60 will undergo 5 half-lives.
To obtain the remaining amount we simplify the expression;
A = 200*(1-0.5)^(26.3/5.26)
= 200*(0.5)^(5)
= 6.25 grams.
Therefore, the amount of a 200 g sample of colbalt-60 that will remain after 26.3 years is 6.25 grams
