A radioactive isotope X with a half-life of   years decays to Y which is stable. A sample of rock from the moon was found to contain both the elements X and Y which were in the ratio of 1 : 7. The age of the rock is

If in the rock there is no Y element intially, then the time taken by element X to reduce to 18th the initial value will be equal to or n =3Therefore, from the beginning three half life time is spent. Hence the age of the rock is = 3x1.37x109 = 4.11x109 years