Question

A certain substance decays to $\frac{1}{32}$ of its initial activity in $25$ days. The half life is:

• A. $1$ day
• B. $3$ days
• C. $5$ days
• D. $7$ days

Answer 1

Answer: (C)

$5$ days

Let the half life be $\lambda$ days.

Substance decays to $1/2$ of it in $\lambda$ days.

Substance decays to $1/4$ of it in $2\lambda$ days.

Similarly Substance decays to $1/32$ of it in $5\lambda$ days. [i.e. 25 days]

$5\lambda =25$

$\lambda =5days$