Question

What Percent of original radioactive substance is left after 5 half life time 

(A) 3% 

(B) 5% 

(C) 6% 

(D) 12%

Answer 1

The correct option is (A) 3%.

Explanation:

Amount of fraction left after time t = (N / N₀) = (1 / 2)^{[t / {T(1/2)}]}

Given : t = 5 half life time = 5T_(1/2)

 ∴ (N / N₀) = (1/2) ^{[{5T(1/2)} / {T(1/2}]} = (1 / 2)⁵

(N / N₀) = (1 / 32)

 ∴ N = [(100) / (32)] % = 3.125% ≈ 3%.