1.  Two trains start from P and Q respectively and travel towards each other at a speed of 50 km/hr and 40 km/hr respectively. By the time they meet, the first train has travelled 100 km more than the second. The distance between P and Q is :
A. 500 km
B. 630 km
C. 660 km
D. 900 km

Answer: Option D

Explanation:

At the time of meeting, let the distance travelled by the the second train be x km.
Then, distance covered by the first train is (x + 100) km.
∴ x /40 = (x + 100) /50

<=> 50x = 40x + 4000
<=> x = 400.

So distance between P and Q = (x + x + 100) km
= 900 km.