The count rate of a Geiger Muller counter for the radiation of a radioactive material of half-life `30` min decreases to `5 s^(-1)` after `2 h`. The initial count rate was

Options

A. `25 s^(-1) `
B. `80 s^(-1)`
C. `625 s^(-1)`
D. `20 s^(-1)`

Answer

B

Explanation

`R= R_(0) ((1)/(2))^((t)/(T_(1//2)))`
`rArr 5= R_(0) ((1)/(2))^((2 xx 60)/(30)) rArr R_(0)= 80 "sec"^(-1)`