One relationship, three rearrangements
The core relationship is distance = speed × time. Knowing any two of the three values lets you solve for the third: distance ÷ speed gives time, and distance ÷ time gives speed. It's the same underlying formula each time, just rearranged around whichever value is missing.
A worked example solving all three directions
Start with a trip covering 240 km at a steady 80 km/h. Solving for time gives 240 ÷ 80 = 3 hours. Now flip it around: if you know the trip took 3 hours at 80 km/h, solving for distance gives 80 × 3 = 240 km — the same 240 km you started with. And if you know the trip covered 240 km in 3 hours, solving for speed gives 240 ÷ 3 = 80 km/h — back to the original speed. All three directions are internally consistent because they're the same relationship viewed from different starting points.
Two more worked examples, one per remaining direction
Solving for distance: 4.5 hours at a steady 60 km/h covers 60 × 4.5 = 270 km. Solving for speed: covering 300 km in 5 hours means averaging 300 ÷ 5 = 60 km/h. Each example uses whichever two values are known to isolate the third — there's no need to memorize three separate formulas, just the one relationship rearranged.
Why "average speed" matters for this formula
This formula works with average speed, not the speed shown on a speedometer at any single moment — a trip with periods of highway driving and periods of city traffic still has one average speed for the whole trip, and that's the value this formula uses. Solving for time or distance using a peak or typical speed instead of the true trip average will produce a misleading result, especially on longer routes with mixed driving conditions.
Where this shows up in everyday trip planning
Estimating how long a drive will take, figuring out how far you can get in the time available, or checking whether an average speed claim is realistic for a known distance and duration are all the same calculation from different starting points — which is why this one relationship, rather than three separate ones, is worth understanding directly rather than looking up each direction separately.