A radio-active material is reduced to $1 / 8$ of its original amount in $3$ days. If $8 \times 10^{-3}\,kg$ of the material is left after $5$ days. The initial amount of the material is $.......\,g$
$64$
$40$
$32$
$256$
Which of the following is not a mode of radioactive decay
Calculate the time (in $minutes$) interval between $33 \,\%$ decay and $67\, \%$ decay if half-life of a substance is $20\, minutes.$
$37$ Rutherford equals
The half life of a radioactive isotope $X$ is $50$ years. It decays to another element $Y$ which is stable. The two elements $X$ and $Y$ were found to be in the ratio of $1 : 15$ in a sample of a given rock. The age of the rock was estimated to be..........$years$
Half lives of two radioactive substances $A$ and $B$ are respectively $20$ minutes and $40$ minutes. Initially the sample of $A$ and $B$ have equal number of nuclei. After $80$ minutes, the ratio of remaining number of $A$ and $B$ nuclei is