40km = 24m
------- x -------
Xkm = 60m
we cross multiply 40x 60 then divide by 24
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100 kmh +/- 5 kmh
24 min = +/- 1.2 min
so...
105 kmh @ 25.2 plus
95 kmh @ 25.2
105 kmh @ 23.8
95 kmh @ 23.8
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400 kmh @ 98
divide by 4 and you have
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100 kmh (@ 24.5 min)