Amar drives his car for 2 hours at a speed of 70 km/h, for 3 hours at a speed of 80 km/h and for 1 hour at a speed of 40 km/h and reaches his hometown. What is his average speed (in km/h)?

We know, 

$$dis\tan ce=speed\ \times\ time$$

Let the distance covered in 2 hrs at the speed of 70 km/hr is $$D_1$$

i.e; $$D_1=70\times\ 2=140 km$$

Let the distance covered in 3 hrs at the speed of 80 km/hr is $$D_2$$

$$D_2=80\times\ 3=240 km$$

Let the distance covered in 1 hr at the speed of 40 km/hr is $$D_3$$

$$D_3=40\times\ 1=40\ km$$

Now, 

Average Speed = $$\frac{Total\ dis\tan ce}{Total\ time\ taken}$$

i.e; $$\frac{\left(140+240+40\right)}{2+3+1}$$

i.e; $$\frac{420}{6}=70\ $$ km/hr

Hence, Option B is correct.