Question 1094371
Travel Rates Rule, RT=D to relate speed, time, distance


r, speed of boat when not in a current


{{{r-2}}}, upstream rate
{{{r+2}}}, downstream rate

total time, 4 hours

<pre>
               SPEED       TIME       DISTANCE
UPSTREAM        r-2       18/(r-2)      18
DOWNSTREAM      r+2       21/(r+2)      21
TOTAL                      4
</pre>

Total time:
{{{18/(r-2)+21/(r+2)=4}}}


.

.
{{{4r^2-39r-10=0}}}
-
{{{discriminant=39^2+4*4*10=1681=41^2}}}
-
{{{r=(39+- 41)/8}}}-----<i>must take the positive value</i>


{{{r=80/8}}}


{{{highlight(r=10)}}}