Question

A radioisotope X with a half life 1.4 x 10⁹ years decays to 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

(a) 1.96 x 10⁹ years

(b) 3.92 x 10⁹ years

(c) 4.20 x 10⁹ years

(d) 8.40 x 10⁹ years

Answer 1

The correct option is (c) 4.20 x 10⁹ years.

Explanation:

So three half lives would have been passed.

t = nT_1/2 = 3 x 1.4 x 10⁹ years = 4.2 x 10⁹ years

Hence. the age of the rock is 4.2 x 10⁹ years.