Question 827271
Set x = speed without current
set d = distance
Going downstream
 rate * time = distance

 (x+3)15 = d so 15x + 45 = d
 (x-3)20 = d, 20x - 60 = d
Since 15x + 45 = d and 20x - 60 = d
 15x + 45 = 20x - 60
subtract 15x from each side
  45 = 5x - 60
add 60 to each side
 105 = 5x
divide 5 by each side
  21 = x
substitute 21 for x in (x+3)15 = d
 (21+3)(15) = d
 (24)(15) = d
 d = 360