Question 1068609
<pre>
c   speed of stream current --- UNKNOWN in this example
r   speed of paddle boat if no current--- 8
t   time either direction -- UNKNOWN in this example
d   downstream distance ---  4
u   upstream distance  --- 2
</pre>

<pre>
                  SPEED        TIME       DISTANCE
DOWNSTREAM        r+c           t           d
UPSTREAM          r-c           t           u
</pre>

Question asks for c, the river current speed.


Basic constant travel rates rule, RT=D to relate rate, time, distance.


{{{d/(r+c)=u/(r-c)}}}---solve this for c in terms of the other variables.


-
{{{d(r-c)=u(r+c)}}}


{{{dr-dc=ur+uc}}}
{{{dr-ur=uc+dc}}}
{{{dr-ur=c(u+d)}}}


{{{highlight(c=(dr-ur)/(u+d))}}}----substitute the given values into this formula.