Question 831798
c = speed of the current.


_______________speed_______________time_____________distance
UP_____________6-c_________________2__________________d
DOWN___________6+c_________________2/3________________d


Same distance each way.  Rate*Time gives the distance, and both expressions must be equal.
Forty minutes is the same as (2/3) hour.  We can keep all in miles and hours.

{{{(6-c)*2=(6+c)(2/3)}}}
Simplify and solve for c.


{{{2*3(6-c)=2(6+c)}}}
{{{3(6-c)=(6+c)}}}
{{{18-3c=c+6}}}
{{{18=c+3c+6}}}
{{{4c=12}}}
RESULT:  {{{c=3}}}  mph