Solution to: Speedy Sums

Let the distance between the stations of Utrecht and Amersfoort be d km. The four minutes delay is ⁴/₆₀ part of an hour. Now the following holds:

d / 90 + ⁴/₆₀ = d / 70.

Solving this equation gives d = 21, so the distance between the stations of Utrecht and Amersfoort is 21 km.

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