If a train [x + 20] metre long passes a pole in 11 seconds, and cross [ x -20] m long bridge in 20 sec. Find the value of x?
- AOption (a)
200
- BOption (b)
260
- COption (c)
320
- DOption (d)
360
- EOption (e)
240
View answer & explanationHide answer & explanation
Correct answer
200
Why this answer is correct
Shortcut Trick
Since the speed of the train is constant, the ratio of distances covered is equal to the ratio of time taken.
Distance 1 (Pole) = [x + 20] in 11 seconds.
Distance 2 (Bridge) = [x + 20] + [x − 20] = 2x in 20 seconds.
⇒ (x + 20) ÷ 11 = 2x ÷ 20 ⇒ (x + 20) ÷ 11 = x ÷ 10
⇒ 10x + 200 = 11x ⇒ x = 200.
∴ The correct answer is 200.
Alternate Method
Given:
Length of train = [x + 20] m
Time to cross pole = 11 sec
Length of bridge = [x − 20] m
Time to cross bridge = 20 sec
Formula Used:
Speed = Distance ÷ Time
Calculations:
⇒ Speed = Length of Train ÷ Time to cross pole
⇒ Speed = (x + 20) ÷ 11 --- (i)
⇒ While crossing a bridge, total distance = Length of Train + Length of Bridge
⇒ Speed = [(x + 20) + (x − 20)] ÷ 20
⇒ Speed = 2x ÷ 20 = x ÷ 10 --- (ii)
⇒ Equating (i) and (ii):
⇒ (x + 20) ÷ 11 = x ÷ 10
⇒ 10(x + 20) = 11x
⇒ 10x + 200 = 11x
⇒ x = 200
∴ The correct answer is 200.
Additional Information
Train Crossing a Point Object
When a train crosses a pole, tree, or stationary person, the distance covered is equal to the length of the train.
Train Crossing a Platform/Bridge
When a train crosses a platform or bridge, the total distance covered is the sum of the length of the train and the length of the platform/bridge.
Unit Conversion
To convert km/hr to m/s, multiply by 5/18. To convert m/s to km/hr, multiply by 18/5.
