Time required for $90\%$ completion of a certain first order reaction is $\mathrm{t}$. The time required for $99.9\%$ completion will be

For the reaction,

${\mathrm{CH}}_3\mathrm{Br}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)⟶{\mathrm{CH}}_3{\mathrm{OH}}^{\ominus }\left(\mathrm{aq}\right)+{\mathrm{Br}}^{\ominus }\left(\mathrm{aq}\right)$, rate law is rate $=\mathrm{k}\left[{\mathrm{CH}}_3\mathrm{Br}\right]\left[{\mathrm{OH}}^{\ominus }\right]$

How does reaction rate changes if $\left[{\mathrm{OH}}^{\ominus }\right]$ is decreased by a factor of $5$?
 

For the reaction,

${\mathrm{CH}}_3\mathrm{Br}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)⟶{\mathrm{CH}}_3{\mathrm{OH}}^{\ominus }\left(\mathrm{aq}\right)+{\mathrm{Br}}^{\ominus }\left(\mathrm{aq}\right)$, rate law is rate $=\mathrm{k}\left[{\mathrm{CH}}_3\mathrm{Br}\right]\left[{\mathrm{OH}}^{\ominus }\right]$

What is change in rate if concentrations of both reactants are doubled?