Question 60884
Let the distance travelled upstream = x km
Speed while going upstream = 4 km/h

So time taken = distance/speed
              = (x/4)h

Speed while travelling downstream = 12 km/h

So time taken = distance/speed
              = (x/12)h [distance upstream =  distance downstream]

Total time taken = 12 h

==> x/4 +  x/12 = 4

The LCD for the left side = 12
==> (3x + x)/12 = 4
==> 4x/12 = 4
multiplying both the sides by 12 we get..
==> 4x = 48
==> 4x/4 = 48/4
==> x = 12

So the distance upstream = 12 km

Good Luck!!!