A distance of 425 km separates two trains moving towards each other at a speed of 200 km/hr each. What will be the distance between them after 1 hr 30 min, if they reduce their speed by half, every half an hour ?

A. 75 km

B. 120 km

C. 150 km

D. 200 km

Answer: Option A

Solution(By Examveda Team)

Relative speed = (200 + 200) km/hr = 400 km/hr
Distance covered in 1 hr 30 min
$$\eqalign{ & = \left( {400 \times \frac{1}{2} + 200 \times \frac{1}{2} + 100 \times \frac{1}{2}} \right){\text{km}} \cr & = \left( {200 + 100 + 50} \right){\text{km}} \cr & = 350{\text{ km}} \cr} $$
[$$\because $$ Speed reduces by half every half an hour]
Hence, distance between the trains :
= (425 - 350) km
= 75 km