For which of the following reactions is the half-life period independent of the initial concentration of the reactant?

Calculate the rate constant of the first order reaction if $20 \%$ of the reactant decomposes in $15 \ minutes$.

$A$ flask is filled with equal moles of $A$ and $B$. The half-lives of $A$ and $B$ are $100 \, s$ and $50 \, s$ respectively and are independent of the initial concentration. The time required for the concentration of $A$ to be four times that of $B$ is $.... \, s.$
(Given : $\ln 2 = 0.693$ )

$R \rightarrow P$ is a first order reaction. For this reaction,a graph of $\ln [R]$ (on $y$-axis) and time (on $x$-axis) gives a straight line with a negative slope. The intercept on the $y$-axis is equal to ($k =$ rate constant):

Consider the two different first order reactions given below:
$A + B \rightarrow C$ (Reaction $1$)
$P \rightarrow Q$ (Reaction $2$)
The ratio of the half-life of Reaction $1$ : Reaction $2$ is $5 : 2$. If $t_1$ and $t_2$ represent the time taken to complete $2/3$ and $4/5$ of Reaction $1$ and Reaction $2$,respectively,then the value of the ratio $t_1 : t_2$ is $. . . . \times 10^{-1}$ (nearest integer).
[Given: $\log_{10}(3) = 0.477$ and $\log_{10}(5) = 0.699$]

For a first order reaction $A \to \text{products}$,the concentration of $[A]$ is reduced from $2 \ M$ to $0.125 \ M$ in one hour. The $t_{1/2}$ of this reaction (in $\text{min}$) is: