Question 111165

Let the speed of the boat be x mph
speed of the current = 4mph

so speed against the current = (x-4)mph
Distance traveled against the current = 20 miles
So time taken = distance/speed
              = 20/(x-4)

Now speed with the current = (x+4)mph
Distance traveled with the current = 60 miles
So time taken = distance/speed
              = 60/(x+4)

As the time taken is the same,

   20/(x-4) = 60/(x+4)

Multiplying both the sides by (x-4)(x+4),

20(x+4) = 60(x-4)

==> 20x + 80 = 60x - 240

==> 240 + 80 = 60x - 20x [ collecting the like terms to one side]

==> 320 = 40x
==> 320/40 = x
==> 8 = x

Thus the speed of the boat in still water = 8mph

Best of luck!!!