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 ?

Answer: Option A

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