Answer
$160$ km, or (H)
Work Step by Step
Go by the formula: $d = rt$, meaning distance = rate*time.
We have two steps with the same distance d, so the total distance is $d+d = 2d$.
Each step has two different rates $r_1, r_2$ that take two different times $t_1, t_2$
$t_1 = 4, t_2 = 10$
If trip 2 is upstream of a 6 km/h current that trip 1 is downstream of, $r_1 = r_2 + 12$
Solve for the rates by setting the distances equal
$d_1 = d_2$, $r_1t_1 = r_2t_2$, $(r_2+12)*4 = r_2*10, \ 4r_2+48 = 10r_2, 6r_2 = 48, r_2 = 8$
$r_1 = r_2+12 = 8+12 = 20$
$d_tot = d_1+d_2 = r_1t_1 + r_2t_2 = 20*4 + 8*10 = 160$
