A man travels 600 km by train at 80 km/hr, 800 km by ship at 40 km/hr, 500 km by aeroplane at 400 km/hr and 100 km by car at 50 km/hr. What is the average speed for the entire distance :

Total distance travelled :
= (600 + 800 + 500 + 100) km
= 2000 km
Total time taken :
$$\eqalign{ & = \left( {\frac{{600}}{{80}} + \frac{{800}}{{40}} + \frac{{500}}{{400}} + \frac{{100}}{{50}}} \right){\text{ hrs}} \cr & = \frac{{123}}{4}{\text{ hrs}} \cr} $$
∴ Average speed :
$$\eqalign{ & = \left( {2000 \times \frac{4}{{123}}} \right){\text{ km/hr}} \cr & = \left( {\frac{{8000}}{{123}}} \right){\text{ km/hr}} \cr & = 65\frac{5}{{123}}{\text{ km/hr}} \cr} $$