Shortcut Trick
Ratio of speeds = 20 : 30 = 2 : 3
Ratio of time taken = 3 : 2 (since distance is constant)
Difference in time ratio = 3 − 2 = 1 unit
Total time difference = 1.5 hours (late start) + 2.5 hours (early arrival) = 4 hours
⇒ 1 unit = 4 hours, so time taken by Car 1 = 3 × 4 = 12 hours
Distance = Speed × Time = 20 × 12 = 240 km
∴ The correct answer is 240 km.
Alternate Method
Given:
Speed of Car 1 (S1) = 20 km/h
Speed of Car 2 (S2) = 30 km/h
Car 2 starts 1.5 hours later and reaches 2.5 hours earlier.
Point A
Point B
Total Distance = D km
Car 1: 20 km/h
Time = t hrs
Car 2: 30 km/h
Time = t − 4 hrs
Formula Used:
Time = Distance ÷ Speed
Difference in time = Late start time + Early arrival time
Calculations:
⇒ Let the total distance between A and B be D km.
⇒ Time difference = 1.5 hours + 2.5 hours = 4 hours.
⇒ Time taken by Car 1 − Time taken by Car 2 = 4 hours
⇒ (D ÷ 20) − (D ÷ 30) = 4
⇒ (3D − 2D) ÷ 60 = 4
⇒ D ÷ 60 = 4
⇒ D = 60 × 4 = 240 km.
∴ The correct answer is 240 km.
Additional Information
Direct Distance Formula
When travelling the same distance at two different speeds S1 and S2 with a known total time difference Δt, Distance = (S1 × S2 ÷ |S1 − S2|) × Δt.
Average Speed
If a vehicle covers equal distance segments at two different speeds S1 and S2, the Average Speed over the entire journey = (2 × S1 × S2) ÷ (S1 + S2).
Relative Speed
When two bodies move in the same direction, their Relative Speed is the difference of their speeds (S1 − S2). If they move in opposite directions, it is the sum of their speeds (S1 + S2).