.
Their relative speed is 100 km/h - 90 km/h = 10 km/h, which means that the bus B runs away from the bus A at the rate of 10 km/h.
Therefore, the two buses will be 10 km apart in 1 hour, i.e. in 60 minutes.
In algebra language, the distance between the buses is 100*t - 90*t kilometers,
and it will be 10 kilometers at the time t
100t - 90t = 10, or
10t = 10
t = = 1 hour = 60 minutes. ANSWER
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