If $t_{1/2}$ is the half life of a substance then $t_{3/4}$ is the time in which substance

Half lives of two radioactive nuclei $A$ and $B$ are $10\, minutes$ and $20\, minutes$, respectively. If, initially a sample has equal number of nuclei, then after $60$ $minutes$ , the ratio of decayed numbers of nuclei $A$ and $B$ will be

Give the equation form of exponential law. 

The half-life of radium is about $1600$ years. Of $100\, g$ of radium existing now, $25\, g$ will remain unchanged after .......... $years$

The half life of a radioactive substance is $20$ minutes. The approximate time interval $(t_2 - t_1)$ between the time $t_2$ when $\frac{2}{3}$ of it had decayed and time $t_1$ when $\frac{1}{3}$ of it had decayed is ..........$min$

After $280$ days, the activity of a radioactive sample is $6000\, dps$. The activity reduces to $3000\, dps$ after another $140\, days$. The initial activity of the sample in dps is