Unit 9 · Speed and Time Calculation
Slow Train vs Express — Find the Departure Time
Lesson 1 of 2
Speed and time are inverses on the same route: travel four times as fast and the journey takes one-quarter as long. Nail that before touching the clock.
Relative-speed timetable questions appear regularly on Selective TS — often as an early item when you translate a speed ratio into minutes on the clock.
The examiner checks whether you can find one vehicle’s trip time, scale it by the speed ratio, then count backwards from a fixed arrival to get the correct departure.
A slow service takes a known time; a faster service uses the same route at a stated multiple of speed. Both must arrive by the same deadline — find when the slower (or faster) one must leave.
Best approach: Step 1: work out the faster train’s elapsed time. Step 2: apply the ratio (4× speed → ÷4 time, etc.). Step 3: subtract from the arrival time. Draw a timeline if AM/PM is tricky.
Question
A slow train leaves Millfield at 7:00 am and reaches Braxton at 1:00 pm the same day. An express train uses the same route at four times the average speed of the slow train. On another day the express leaves Millfield later but still arrives in Braxton at 1:00 pm.
Both trains run at constant speed without stopping.
At what time did the express train leave Millfield?
Select an answer to see the explanation.
Give Your Child the Best Chance at Selective Entry
Join NSW families preparing their children for the Selective Schools Placement Test with the most realistic online Selective practice tests available. First tests free—no credit card required.
Claim Your Free Selective Practice Tests