The half-life of a radioactive isotope X is 50 yr.
It decays to another element Y which is stable.
The two elements X and Y were found to be in
the ratio of 1:15 in a sample of a given rock.
The age of rock was estimated to be
(a) 200 yr
(b) 250 yr
(c) 100 yr
(d) 150 yr