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A train 800 meters long is running at the speed of 78 kmph. If it crosses a tunnel in 1 minutes, then the length of the tunnel (in meter) is:
1. 1500
2. 77200
3. 500
4. 13

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Correct Answer - Option 3 : 500

Given:

Length of train = 800 m

Speed of train = 78 km/h

Time to cross the tunnel = 1 minute = 60 seconds

Concept used:

1 km/h = 5/18 m/s

Formula used:

Speed = (Length of train + Length of tunnel)/Time

Calculations:

Speed = (Length of train + Length of tunnel)/Time

⇒ 78 × (5/18) = (800 + x)/60

⇒ 65/3 = (800 + x)/60

⇒ 65 × 20 = 800 + x

⇒ 1300 = 800 + x

⇒ x = 500 m

∴ The length of the tunnel is 500 m.

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