Question 203740
S=D/T
Therefore D= S x T
For the first cyclust we have him traveling at 9mph.. and after 90mins where is he... 
90 mins = 1 hour and half. or 1.5 hours. 

D= 9mph x 1.5 hours
  = 13.5 miles.

Second cyclist is travelling at 4mph... and after 90 mins where is he.. 
90 mins = 1.5 hours.

D= 4mph x 1.5 hrs
  = 6 miles.
How far apart are they? We need to find the differance which is like this..
first biker-second biker= distance apart 
13.5 miles-6 miles= 7.5 miles
Therefore the bikers are 7.5 miles apart or the first biker is 7.5 miles ahead of the second biker after 90 mins. 

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