Give the equation form of the exponential law of radioactive decay.

Starting with a sample of pure ${}^{66}Cu$,$\frac{7}{8}$ of it decays into $Zn$ in $15 \ minutes$. The corresponding half-life is .......... $minutes$.

$A$ radioactive sample has a half-life of $5$ years. The probability of decay in $10$ years will be ........$\%$

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 the remaining numbers of $A$ and $B$ nuclei is

The activity of a radioactive material is $2.56 \times 10^{-3} \, Ci$. If the half-life of the material is $5 \, \text{days}$, after how many days will the activity become $2 \times 10^{-5} \, Ci$?

Plutonium decays with a half-life of $24000 \, \text{years}$. If the plutonium is stored for $72000 \, \text{years}$, then the fraction of plutonium that remains is