Question

A mixture consists of two radioactive materials $A_1$ and $A_2$ with half lives of 20s and 10s respectively . Initially the mixture has 40 g of $A_1$ and 160g of $A_2$ . The amount of the two in the mixture will become equal after

• A. 60s
• B. 80s
• C. 20s
• D. 40s

Answer 1

The correct option is D 40s

For $40g$ amount

$4g\underset{half-life}{\overset{20s}{\to }}20g\stackrel{20s}{\to }10g$

For $160g$ amount

$160g\stackrel{10s}{\to }80g\stackrel{10s}{\to }40g\stackrel{10s}{\to }20g\stackrel{10s}{\to }10g$

So after $40s$, $A_1$ and $A_2$ remains same.