Two trains are moving in opposite directions at $29\text{ km/h}$ and $25\text{ km/h}$. The faster train crosses a man sitting in the slower train in 6 seconds. Find the length of the faster train.

Since the two trains are moving in opposite directions, their relative speed is the sum of their individual speeds:
$$\text{Relative Speed}=29+25=54\text{ km/h}$$

Converting this speed into meters per second ($\text{m/s}$):

$$54\times \frac{5}{18}=15\text{ m/s}$$

When the faster train crosses a man in the slower train, the distance covered equals the length of the faster train:

$$\text{Length}=\text{Relative Speed}\times \text{Time}=15\text{ m/s}\times 6\text{ s}=90\text{ m}$$