What is the unit of the rate constant for a first-order reaction?

The half-life period of a first order reaction is $100 \, \text{sec}$. The rate constant of the reaction is

Derive the equation showing the relation between the concentration $[R]_1$ and $[R]_2$ at time $t_1$ and $t_2$ for a first-order reaction.

In a first order reaction,$87.5 \%$ of the reactant is converted into the product in $15 \ minutes$. The rate constant for the reaction is given by:

In a first order reaction,the concentration of the reactant decreases from $20 \ mmol$ to $10 \ mmol$ in $1.151 \ min$. What is the rate constant (in $min^{-1}$)?

How much time is required for a first-order reaction to be $3/4$ complete?