The "time" equation is
      =  ,
where " u " is the paddling rate in still water.
From the equation
    12*(u + 2) = 28*(u-2)
    12u + 24   = 28u - 56
    24 + 56 = 28u - 12u
    80      = 16u
    u       = 80/16 = 5.
ANSWER.  Their paddling rate in still water is 5 miles per hour.