The acticity of a sample is `64 xx 10^-5 Ci`. Its half-life is `3` days. The activity will become `5 xx 10^-6 C i` after.

Options

A. 12 days
B. 7 days
C. 18 days
D. 21 days

Answer

D

Explanation

(d) `A = A_0 ((1)/(2))^(t//T_1//2) rArr 5 xx 10^-6 = 64 xx 10^-5 ((1)/(2))^(t//3)`
`(1)/(128) = ((1)/(2))^(t//3) rArr t = 21 days`.