Abus averages 6 miles per hour faster than a trolley. if the bus travels 90 miles in the same time it takes the trolley to travel 75 miles, then what is the speed of each?

The first thing we are going to do for this case is to define a variable.
 x: speed of the trolley
 Then, by definition we have to:
 
 Where,
 t: time
 d: distance
 v: speed
 For the bus we have:
 
 For the trolley we have:
 
 Since the time is the same then we have:
 
 From here, we clear the value of x:
 
 So, the speed of the trolley is:
 
 The bus speed is:
 
 
 Trolley: 18 m / h
 Bus: 24 m / h

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