\[(a)\; 5 m/s \quad (b)\;4 m/s \quad (c)\;4.5 m/s \quad (d)3.5m/s \]

Let total distance 's'

Let time coverd for first one third distance be $t_1$

then $t_1=\large\frac{s/3}{4}=\large\frac{s}{12}$

$t_2$ be time for next two journeys

$\large \frac{2s}{3}$$=2t_2+6 t_2=8t_2$

$t_2=\large\frac{s}{12}$

$average\;velocity=\large\frac{Total\;distane}{Total\;time}$

$\qquad=\large\frac{s}{t_1+2 t_2}$

$\qquad=\large\frac{s}{\Large\frac{s}{12}+\frac{2s}{12}}$

$\qquad=4 m/s$

Hence b is the correct answer

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