Nylon-$66$ is not a ......

The half-life of a radioactive substance against $\alpha$-decay is $1.2 \times 10^7 \ s$. What is the decay rate for $4 \times 10^{15}$ atoms of the substance?

Who was the first scientist to use the term 'virus' as a pathogen of infectious diseases?

$A$ three-digit number $n$ is such that the last two digits are equal and differ from the first digit. The number of such $n$'s is:

The extent of charge of a lead accumulator is determined by

$A$ and $B$ are two radioactive substances whose half-lives are $1$ and $2$ years respectively. Initially, $10 \, g$ of $A$ and $1 \, g$ of $B$ are taken. The time (approximate) after which they will have the same quantity remaining is ........... $years$.