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 1:7 in a sample of a given rock. The age of the rock is estimated to be

• A. 40 years
• B. 60 years
• C. 80 years
• D. 100 years

Answer 1

Answer: (B)

60 years

$\frac{N}{N_0}=\frac{1}{8}=\frac{1}{2^3}$

$3$ half lives, $T=3\times 20=60$ years