Question

The half-life of a radioactive isotope `X` is `20` years. It decays to another element `Y` which is stable. The two elements `X` and `Y` were found to be in the ratio of `1 : 7` in a sample of a given rock. The age of the rock was estimated to be.
A. 40 years
B. 60 years
C. 80 years
D. 100 years

Answer 1

Correct Answer - B
(b) As `(N_x)/(N_y) = (1)/(7)` (Given)
`rArr (N_x)/(N_x + N_y) = (1)/(8) = ((1)/(2))^3`
`rArr 3` half-lives
Therefore, age of the rock
`t = 3T_(1//2) = 3 xx 20 yrs = 60 yrs`.