Neet Solved Paper 2013 Question 35
Question: The half-life of a radioactive isotope X is 20 yr. It decays to another element Y which is stable. The two elements X and Y were found to be in the ratio 1:7 in a sample of a given rock. The age of the rock is estimated to be
Options:
A) 40 yr
B) 60 yr
C) 80 yr
D) 100 yr
Show Answer
Answer:
Correct Answer: B
Solution:
- As $ \frac{N}{N _0}={{( \frac{1}{2} )}^{n}} $ $ \frac{N}{N _0}={{( \frac{1}{2} )}^{3}}=\frac{1}{8} $ Number of half lives = 3
$ \Rightarrow $ $ T=20yr $
$ \therefore $ $ T=\frac{t}{n} $ or $ t=T\times n=20\times 3yr=60yr $
