The half-life of a radioactive isotope `X` is `20 yr`. It decays to another element `Y` which is stable. The two elements `X` and `Y` were found to be in the ratio `1:7` in a sample of given rock. The age of the rock is estimated to be

Options

A. 40 yr
B. 60 yr
C. 80 yr
D. 100 yr

Answer

B

Explanation

As `(N)/(N_(0))=((1)/(2))^(n)=(N)/(N_(0))=((1)/(2))^(3)=(1)/(8)`
Number of half-lives=3
`implies T=20` yr
`therefore T=(t)/(n)`
or `t=Txxn=20xx3yr=60yr`