A train started from station A and travelled towards station B at 70 km/h. At the same time another train started from station B and started travelling to station A at 60 km/hr. After 11/2 hours they were still 65 km away from each other and had not crossed each other. After how many hours of starting would they cross each other?

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  1. 12
  2. 10
  3. 8
  4. 6

Answer (Detailed Solution Below)

Option 4 : 6
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Given:

Speed of train from A = 70 km/h

Speed of train from B = 60 km/h

Time = 11/2 hours

Distance between them after 11/2 hours = 65 km

Formula Used:

Distance = Speed × Time

Relative Speed = Speed1 + Speed2 (when moving towards each other)

Calculation:

Distance covered by train from A in 11/2 hours = 70 × 11/2 =  385 km

Distance covered by train from B in 11/2 hours = 60 × 11/2 = 330 km

Total distance covered by both trains = 385 + 330 = 715 km

Total distance between station A and B = Distance covered + Distance remaining

Total distance between station A and B = 715 + 65 = 780 km

Relative speed of the trains = 70 + 60 = 130 km/h

Time to cross each other = Total distance / Relative speed

⇒ Time = 780 / 130 = 6 hours

∴ The trains would cross each other after 6 hours of starting.

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