Shortcut Trick
Total distance = 150 + 250 = 400 m.
Relative speed in same direction = S1 − S2 = 400 ÷ 20 = 20 m/s.
Relative speed in opposite direction = S1 + S2 = 400 ÷ 5 = 80 m/s.
Solving, S1 = (80 + 20) ÷ 2 = 50 m/s and S2 = (80 − 20) ÷ 2 = 30 m/s.
Speeds in km/h = 50 × (18/5) = 180 km/h and 30 × (18/5) = 108 km/h.
∴ The correct answer is 180 km/h and 108 km/h.
Alternate Method
Given:
Length of first train (L1) = 150 m
Length of second train (L2) = 250 m
Time taken to cross in same direction (T1) = 20 seconds
Time taken to cross in opposite direction (T2) = 5 seconds
Formula Used:
Speed = Distance ÷ Time
Total distance covered to cross each other = L1 + L2
Calculations:
⇒ Total Distance = 150 + 250 = 400 m
⇒ Let the speed of the first train be u m/s and the second train be v m/s.
⇒ When running in the same direction, relative speed = u − v
⇒ u − v = 400 ÷ 20 = 20 m/s ... (Equation 1)
⇒ When running in opposite directions, relative speed = u + v
⇒ u + v = 400 ÷ 5 = 80 m/s ... (Equation 2)
⇒ Adding Equation 1 and Equation 2:
⇒ 2u = 100 ⇒ u = 50 m/s
⇒ Subtracting Equation 1 from Equation 2:
⇒ 2v = 60 ⇒ v = 30 m/s
⇒ Convert speeds to km/h by multiplying with (18/5):
⇒ Speed of first train = 50 × (18/5) = 180 km/h
⇒ Speed of second train = 30 × (18/5) = 108 km/h
∴ The correct answer is 180 km/h and 108 km/h.
Additional Information
Relative Speed
When two objects move in opposite directions, their relative speed is the sum of their individual speeds (S1 + S2). When moving in the same direction, it is the difference of their speeds (|S1 − S2|).
Distance While Crossing
When two trains cross each other, the total distance covered is always the sum of their lengths (L1 + L2), regardless of the direction they are moving.
Unit Conversion Trick
To rapidly convert speed from m/s to km/h, multiply the value by 18/5. To convert from km/h to m/s, multiply by 5/18.