Time conversion:   15 minutes =  15/60 hour = 0.25 hour

To amusement park:  let  t  represent time

                                          d = rt = 50t

From amusement park:   t - 0.25 represents time

                                           d = rt = 60(t - 0.25)

The two distance are equal:

                      50t  =  60(t - 0.25)

                      50t  =  60t - 15

                       15  =  10t

                       1.5  =  t

      d = 50(1.5) = 60(1.5 - 0.25) = 75 miles

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