The half life of a radioactive substance is `20` minutes . The approximate time interval `(t_(2) - t_(1))` between the time `t_(2)` when `(2)/(3)` of it had decayed and time `t_(1)` when `(1)/(3)` of it had decay is

Options

A. 14 min
B. 20 min
C. 28 min
D. 7 min

Answer

B

Explanation

`therefore N_(1) = N_(0)-1/3N_(0)=2/3N_(0)` and `N_(2)=N_(0)-2/3N_(0)=1/3N_(0)`
`therefore N_(2)/N_(1) = (1/2)^(n) rArr n=1`
`therefore t_(2)-t_(1)` = one half-life = 20 min