Number of nuclei of a radioactive substance at time $t = 0$ are $1000$ and $900$ at time $t = 2 \, s$. Then number of nuclei at time $t = 4 \, s$ will be

The half-life of radioactive $Po$ (Polonium) is $138.6 \, \text{days}$. For $1,000,000$ Polonium atoms, the number of disintegrations in $24 \, \text{hours}$ is:

Half-life of a substance is $20 \, minutes$. What is the time between $33 \%$ decay and $67 \%$ decay? ................ $minutes$

Define the average life of a radioactive sample and obtain its relation to decay constant and half-life.

$A$ source contains two phosphorus radionuclides $_{15}^{32} P \left(T_{1/2} = 14.3 \ d\right)$ and $_{15}^{33} P \left(T_{1/2} = 25.3 \ d\right)$. Initially,$10\%$ of the decays come from $_{15}^{33} P$. How long must one wait until $90\%$ of the decays come from $_{15}^{33} P$?

In a sample of radioactive material,what fraction of the initial number of active nuclei will remain undisintegrated after half of a half-life of the sample?