Average speed

Total Time Travel: 4/3 + 4/6 = 2 hours. Total distance travel: 40 + 40 = 80. Average Speed would be 80 / 2 = 40 km/h.

Time travel on a straight road for $40 km$ at $30 km$ : $\quad$ $t_{1}=\frac{S_1}{V_1}$

$\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad$ $\Rightarrow t_1=\frac{40}{30}=\frac{4}{3} \quad \quad \quad \quad \left( h \right)$

Similar, Time travel in the same direction for another $40 km$ at $60 km$ : $\quad$ $t_2=\frac{S_2}{V_2}$

$\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad$ $\Rightarrow t_2=\frac{40}{60}=\frac{2}{3} \quad \quad \quad \quad \left( h \right)$

So, Total Time Travel: $\quad t=t_1+t_2 = \frac{4}{3}+\frac{2}{3} = 2 \quad \quad \quad \quad \left( h \right)$

And, Total distance travel: $\quad S=S_1+S_2 = 40 + 40 = 80\quad \quad \quad \quad \left( km \right)$

Thus, The average speed is $\quad V=\frac{S}{t} = \frac{80}{2} = \boxed{40} \quad \quad \quad \quad \quad \quad \left( \frac{km}{h} \right)$

When distance travelled is same then the average speed is:

S =2 (s1 * s2) / (s1 + s2)

for this problem, Average speed S = 2*(30 * 60) / (30 + 60) = 40 km/h

The distances are the same for each section of the journey, so we can simply take the harmonic mean of the speeds:

$\frac{2}{\frac{1}{30}+\frac{1}{60}} = 40$

Yo, as v = s / t , then t = s / v ,

The average speed = (40 km + 40 km) / [ ( 40 km / 30 km/h ) + ( 40 km / 60 km /h ) ] = 80 km / 2 h = 40 km/h....

Thanks....