Fill in the blanks with appropriate items :
A radioactive substance decays 20% in 10 min if at start there are ` 5 xx 10^(20)` atoms present , after ________ hour the number of atoms will be reduced to ` 10^(18)`.

Answer

`4.65`

Explanation

`k=(1)/(10) ln .(100)/(80) =(1)/(t) "in" (5xx10^(20))/(10^(18)) rArr (1)/(10)"in"(5)/(4)=(1)/(t) "in" 500 rArr (10 xx"in" 500)/("ln"(5)/(4))`
`t=278.5` min.
`t=4.641` hr.