# The radioactive isotope X with a half life of 10^9 years decays to Y which is stable. A sample of rocks

Q: The radioactive isotope X with a half life of 109 years decays to Y which is stable. A sample of rocks was found to contain both the elements X and Y in the radio 1∶ 7. What is the age of the rocks ?

(a) 2 × 109 Y

(b) 3 × 109 Y

(c) 6 × 109 Y

(d) 7 × 109 Y

Ans: (b)
Sol: $\displaystyle \frac{N}{N_0} = \frac{1}{1+7}$

$\displaystyle \frac{N}{N_0} = \frac{1}{8} = (\frac{1}{2})^3$

As we know , $\displaystyle \frac{N}{N_0} = (\frac{1}{2})^n$

On comparing we get,

n= 3

t = n T

= 3 × 109 Y