Which of the following is a first order reaction?

If the half-life $(t_{1/2})$ of a first-order reaction is $20 \ minutes$,what fraction of the reactant remains after $40 \ minutes$?

The half-life of a first-order reaction is $850 \text{ s}$. The initial concentration of the reactant is $[R]_0 = 0.06 \text{ mol dm}^{-3}$. What concentration $[R]$ would remain after $t = 1200 \text{ s}$?

For a first order reaction,the half-life period is independent of

$A$ first order reaction is $25 \%$ completed in $40 \ min$. What is the rate constant $k$ for the reaction?

$R \rightarrow P$ 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):