A radio isotope X with a half life 1.4x109 yr decays of Y which is stable. A sample of the rock from a cave was found to contain X and Y in the ratio 1:7. The age of the rock is
Ratio of X:Y is given=1:7
=> Let the initial total mass is m.
=> mx+my=m => my/7+my=m
only 1/8 part remains
T/2 T/2 T/2
So,time taken to become 1/8 unstable part
=3 x T1/2 = 3 x 1.4 x 109 =4.2 x 109 y