A boat takes 4 hours for travelling downstream from point A to point B and coming back to point A upstream. If the velocity of the stream is 2 Kmph and the speed of the boat in still water is 4 kmph, whit is the distance between A and B

d = 6 km

Explanation:

In this case we have the relative speed of the boot changes, suppose that the boat and the river have the same direction when the boat goes from A to point B

       vbr = vrt + vbt

        vbr = 2 + 4

        vbr = 6 km / h

        v₁ = 6 km / h

velocity is constant

         v₁ = d / t₁

         t₁ = d / v₁

when the boat returns to the starting point the speed is both opposite

        vbr = vbt - vrt

        vbr = 4-2

        vbr = 2 km / h

        v₂ = 2 km / h

as the speed is constant

         t₂ = d / v₂

they also indicate that the total time is 4 h

       t₁ + t₂ = 4

we substitute

       4 = d / v₁ + d / v₂

       4 = d (1 / v₁ + 1 / v₂)

       d = 4 / (1 / v₁ + 1 / v₂)

let's calculate

       d = 4 / (1/6 + 1/2)

       d = 4 / 0.6666 67

       d = 6 km

real, smaller and upside-down.   so the answer should be b.