Question

The activity of a radioactive material is 6.4 × 10-4 curie. Its half life is 5 days. The activity will become 5 × 10-6 curie after

• A. 7 days
• B. 15 days
• C. 25 days
• D. 35 days

Answer 1

Answer: (D)

35 days

Answer 2

The correct answer is (D) : 35 Days
$∵ A = \frac{A_0}{\frac{t}{2^{T_{1/2}}}}$
$⇒ 2^{t/5} = \frac{6.4×10^{-4}}{5×10^{-6}}$
$= 128 = 2^7$
$⇒ \frac{t}{5} = 7$
⇒ t = 35 days