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 -

Question

( A ) 60 years
( B ) 40 years
( C ) 80 years
( D ) 100 years