Question 1057326
No; her motorboat does not run at a constant pace.
If you will describe this exercise correctly, I will solve most of it for you.


I see how this SHOULD be written:
Carol requires 3 hours to go downstream 21 miles in her motorboat.  Her return trip upstream requires 3 1/2 hours. Find the speed of her motorboat in still water.


The "assuming" part is wrong.



Let c be the speed of the river current.


SETUP TABLE OF DATA

<pre>
                       speed        time        distance

DOWN                   r+c          3            21

UP                     r-c         {{{3&1/2}}}        21
</pre>


CREATE SYSTEM OF EQUATIONS USING TRAVEL RATE RULE

{{{system((r+c)*3=21,(r-c)(3&1/2)=21)}}}




SOLVE THE SYSTEM FOR THE UNKNOWN VARIABLES
-
{{{system(r+c=7,r-c=21/(7/2))}}}
-
{{{system(r+c=7,r-c=3/2)}}}
-
{{{r+c+r-c=7+3/2}}}
{{{2r=(14+3)/2}}}
{{{highlight(r=17/4)}}}----------and this is what the question asked for.
{{{(4&1/4)(miles/hour)}}}.