A train X travelling at 72 km/h crosses another train Y travelling at 63 km/h (in opposite direction) in 18 seconds. If the length of train Y is two-third the length of X, then length of train X is:

405 m

Relative speed = 72 + 63 = 135 km/h

135 km/h = 135 × \(\frac{5}{18}\) = 37.5 m/sec

Total distance = 37.5 × 18 = 675 m

According to question ,

Let length of train X = 3X

Then Length of train Y = 2X

3X +  2X = 675 m

5X = 675 m

X = 135 m

So , Length of train X = 3 × 135 = 405 m