Question 110682
Let R = Darrell's rowing speed in still waters and C = The current speed. 
 
From the the problem description, you can write:
1) {{{d[1] = r[1]t[1]}}} For the downstream trip.
2) {{{d[2] = r[2]t[2]}}} For the upstream trip.

1) {{{24 = r[1](6)}}}
{{{r[1] = 4}}}
2) {{{18 = r[2](6)}}}
{{{r[2] = 3}}}

The downstream trip rate can be thought of as Darrells's rowing speed plus the current speed, or:
{{{r[1] = R+C}}} or:
{{{4 = R+C}}}
The upstream trip rate can be thought of as Darrell's rowing speed minus the current speed, or:
{{{r[2] = R-C}}} or:
{{{3 = R-C}}}
So you can add these two equations to find the value of R, Darrell's rowing speed in still water:

{{{4 = R+C}}}
{{{3 = R-C}}}
-------------
{{{2R = 7}}}
{{{R = 3.5}}}
Darrell's rowing speed in still water is 3.5 km/hr.
The current speed is:
{{{C = R-3}}}
{{{C = 3.5-3}}}
{{{C = 0.5}}}km/hr