Question

Assertion :If the half life of a radioactive substance is $40days$ then $25$ % substance decay in $20days$. Reason: $N=N_0{\left(\frac{1}{2}\right)}^n$ where, $n=\frac{\text{time elapsed}}{\text{half life period}}$

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Assertion is incorrect but Reason is correct

Answer 1

The correct option is D Assertion is incorrect but Reason is correct

Halflife of radioactive substance is $40days$. It means $50$ % substance decays in $40days$. During this period rate of decay is on decrease. So, $25$% decay must have taken place is less than $20days$.
$N=N_0{\left(\frac{1}{2}\right)}^n$, where $n=\frac{\text{time elapsed}}{\text{half life period}}$