The half-life of a radioactive substance is $3.6$ days. How much of $20\, mg$ of this radioactive substance will remain after $36$ days ............. $mg$

A sample of radioactive element has a mass of $10\, gm$ at an instant $t = 0$.The approximate mass of this element in the sample after two mean lives is ..........$gm$

Half life of a radioactive element is $10\, days$. The time during which quantity remains $1/10$ of initial mass will be .........$days$

Half-life of a radioactive substance is $20$ minutes. Difference between points of time when it is $33\%$ disintegrated and $67\%$ disintegrated is approximately ........... $min$

A sample initially contains only $U -238$ isotope of uranium. With time, some of the $U -238$ radioactively decays into $Pb -206$ while the rest of it remains undisintegrated.

When the age of the sample is $P \times 10^8$ years, the ratio of mass of $Pb -206$ to that of $U -238$ in the sample is found to be $7$ . The value of $P$ is. . . . . .

[Given : Half-life of $U-238$ is $4.5 \times 10^9$ years; $\log _e 2=0.693$ ]

Certain radioactive substance reduces to $25\%$ of its value in $16\ days$. Its half-life is .......... $days$