Question 1046879
This is too common a type of question so I will begin to answer purely in symbols.  Your boat is able to move faster going downstream, but if in still water, assigning the rate or speed r.


<pre>
               SPEED         TIME        DISTANCE

DOWNSTR        r+c            t           D

UPSTRM         r-c            t           d
</pre>

{{{system(VARIABLES,GIVEN,D=80,d=50,t=2.5,UNKNOWNS,r=boatStillWater,c=current)}}}


{{{system(t(r+c)=D,t(r-c)=d)}}}-------two equations in two unknown variables; the others are given and constant knowns.
Solve for c and r.




{{{system(tr+tc=D,tr-tc=d)}}}



{{{tc=D-tr}}}
{{{c=(D-tr)/t}}}
-
{{{tr-t((D-tr)/t)=d}}}
{{{tr-(D-tr)=d}}}
{{{tr-D+tr=d}}}
{{{2tr-D=d}}}
{{{2tr=d+D}}}
{{{highlight(r=(d+D)/(2t))}}}-------formula for the still-water speed



Note, I used substitution, but Elimination might be a possible choice to use.  Elimination being used in finding c, next.



{{{tr+tc-(tr-tc)=D-d}}} just subtraction corresponding members of the two equations of the system
{{{2tc=(D-d)}}}, which immediately seems more efficient a method...
{{{highlight(c=(D-d)/(2t))}}}------river current speed



---
AFTER-THOUGHT

A better way, nearly the same as I showed, would be, fill-in the table with the ACTUAL values first, and of course the UNKNOWN variables too, AND THEN make the generalizations for ALL THE GIVEN VALUES.  Now make a new table, but use only variables for everything.  

NOW you can more comfortably write your system of equations, and keep in mind which variables are the unknowns.  Solve this system purely in variables.  You will have formulas for each unknown variable from that.