Let
x = velocity of the water current
t1 = time taken to go downstream
t2 = time taken to go upstream
t1 + t2 = 8
t2 = 8 – t1
v = D/t
Downstream
(4 + x) = 6/ t1
t1 = 6/(4+x)
Upstream
(4 – x) = 6/t2
Substitute expression for t2 to obtain
(4 – x) = 6/ (8-t1)
Substituting the expression of t1 to make a single variable equation, obtaining
6 = (4-x)*(8 – 6/(4+x))
Solving for x,
x= 3.16 m/s