The railway engine passes two bridges of lengths 400 m and 235 m in 100 seconds and 60 seconds, respectively. Twice the length of the railway engine (in m) is :

 25

Let us assume the length of the engine is = x m

Part1  (Bridge is 400 m long)

 Speed of engine = c= 4 m/sec

Part 2 (Bridge is 235 m long)

 Speed of engine = \(\frac{235+x }{60}\)

Since the speed of the engine is constant, both the speeds should be equal

\(\frac{400+x }{100}\) = \(\frac{235+x }{60}\)

Solving for x  we get-

 2400 + 6x = 2350 + 10x

 4x = 50

x = 12.5

Twice the length of engine = 2 × 12.5 = 25 m

Twice the length of engine is 25m