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 is estimated to be

(a) 60 years

(b) 80 years

(c) 100 years

(d) 40 years

**Ans: (a) 60 years**

**Solution :**