1 answer
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Do read the earlier two first.
At the end of hour five, which we'll call t5,
speed @t5 = 100 km/h
It's been accelerating at 3km/h, so at 4 hours
speed @t4 = 100 - 3 = 97 km/s
and at 3 hours
speed @t3 = 97 - 3 = 94 km/s
You can work it back from there.