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a car and a bus start at 8:00 a.m. from the same point and travel in the same direction. the average speed of the car is 60 mph. 
in 2 hours the car is 30 miles ahead of the bus. find the average speed of the bus.
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Since the average speed of the car was 60 mph (given !), in 2 hours it covered  2*60 = 120 miles.
Since the car was  30 miles ahead the bus at that time moment,
it means that the bus covered  90 = 120 - 30 miles in 2 hours.
Hence, the average speed of the bus was   = 45 miles per hour.
Answer.  The average speed of the bus was  45 mph.
Solved.
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