A local train of length 250 m should cover a distance of 100 km. There is a bridge of length ‘x’ in the journey where the maximum speed should not exceed 25 km/hr.The train used to go at a speed of 75km/hr at other places and 15 km/hr on the bridge. Due to fog, one day its speed reduced by 15 km/hr in other places. Even by travelling at 80% of the maximum speed on the bridge, it was 25 min late. How much time will the train take to cross the bridge usually (in minutes)?

Option 1 : 151

Given, length of the bridge = x

⇒ Distance need to be covered other than in the bridge = 100 - x

If the train travels in the usual speed,

Total time taken = Time taken to cross bridge + Time taken to cover remaining distance

Time = Distance/Speed

Let the total time taken by the train if it travels at usual speed be t

⇒ t = x/15 + (100 – x)/75      ----eq. 1

While the train travels in fog, it was 25 minutes late

⇒ t - 25/60 = x/20 + (100 – x)/60      ----eq. 2

Equation 1 - 2

⇒ 25/60 = x (1/15 – 1/20) +

⇒ 25/60 = x/60 – (100 – x)/300

Solving gives

 x = 37.5 km

Time taken to cross the bridge = (Length of the bridge + Length of the train)/ Speed

= (37.5 + 0.25)/15 = 37.75/15 hours