Question

The activity of a radioactive material is 2.56 × 10–3 Ci. If the half life of the material is 5 days, after how many days the activity will become 2 × 10–5 Ci?

• A. 30 days
• B. 35 days
• C. 40 days
• D. 25 days

Answer 1

Answer: (B)

35 days

Answer 2

The correct option is( B): 35 days.

$A=A_{0}e^{-λt}$

⇒ 2 × 10–5 = 2.56 × 10–3 e –λ t

$⇒e^{-λt}=\frac{1}{128}=[\frac{1}{2}]^7$

$⇒t=7t_{\frac{1}{2}}=35\,days$