Question 384465
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Hi,   
D = r*t      
Let (x+c) and (x-c) represent speed downstream and upstream respectively
Question states
 7.5mi = 1.5hrs(x+c) solving for x
 7.5/1.5 - c = x
     5 -c = x

7.5(x-c) = 2.5hr  
    (x-c) = 2.5/7.5   substituting for x to solve for c
    [(5-c) - c] = 1/3
        5 - 2c = 1/3
        15/3 - 1/3 = 2c
         14/3 = 2c
           c = 7/3 or 2 1/3mph  the speed of the current
           x = 2 2/3mph  the speed of the boat in still water
CHECKING our Answer
  3/2hr * 5mph = 15/2 = 7.5mi