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 :

Answer: Option D

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