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# In a first order reaction,the concentration of the reactant decreases from 0.02M to 0.005M in 10min.The time taken to change the concentration of reactant from 0.16M to 0.025M

$\begin{array}{1 1}(a)\;10min&(b)\;20min\\(c)\;40min&(d)\;30min\end{array}$

Let $t_2$ be the time for reactant to change concentration from 0.16 to .025.
$\large\frac{t_2}{10}=\frac{1/k\;ln 64/[A_0]}{1/k\;ln 4[A_o]}$$=3$
$t_2=30min$
Hence (d) is the correct answer.