Q. 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
Explanation
[Solution from source] X : Y = 1 : 7 means 1/8 of the original X remains. 1/8 = (1/2)³, so three half-lives have passed: 3 × 20 = 60 years. 80 years would leave 1/16. Answer (2).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.