AlgebraVery hardDistance from a time saved on the return
Algebra practice question
A train travels from town A to town B at 80 kilometres per hour and returns at 120 kilometres per hour. The return journey takes 30 minutes less than the outward journey. How far apart, in kilometres, are the two towns?
Answer: 120
d/80 − d/120 = 1/2 hour. With denominator 240: 3d − 2d = 120, so d = 120.
Common wrong answers
- If you answered 40
- 40 subtracts the speeds.
- If you answered 60
- 60 halves the answer, treating the half-hour difference as if it applied to one leg only.
- If you answered 240
- 240 doubles the distance by treating the difference as 1 hour.
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