${t_1} = \frac{{x/2}}{3} = \frac{x}{6}$…(i)
${x_1} = 4.5\;{t_2}$ and ${x_2} = 7.5\;{t_2}$
So, ${x_1} + {x_2} = \frac{x}{2} \Rightarrow 4.5{t_2} + 7.5{t_2} = \frac{x}{2}$
${t_2} = \frac{x}{{24}}$…(ii)
Total time $t = {t_1} + 2{t_2} = \frac{x}{6} + \frac{x}{{12}} = \frac{x}{4}$
So, average speed $ = 4\;m/sec$.