Question 807766
r = boat speed if no current present.
c = 2 km/h current of river.
u = time going upstream
3-u = time going downstream (based on description and on assignment of u)



________________speed____________time___________distance(km)
Upstream________r-c______________(u)__________(___)
Downstream______r+c______________(3-u)__________(___)
Total______________________________3_____________15



Two unknown variables ,  r and u.  Filling in the distance data,

________________speed____________time___________distance(km)
Upstream________r-c______________(u)__________((r-c)(u))
Downstream______r+c______________(3-u)__________((r+c)(3-u))
Total______________________________3_____________15



The "Total" data gives an equation:
{{{(r-c)(u)+(r+c)(3-u)=15}}}
'
Steps
{{{ru-rc+3r+3c-ur-uc=15}}}
{{{ru-rc+3r-r+3c-uc=15}}}
{{{r(u-c+3-1)+3c-uc=15}}}
{{{r(u-c+2)=15+uc-3c}}}
{{{r=(15+uc-3c)/(u-c+2)}}}
Substituting for c=2,
{{{r=(15+2u-6)/(u)}}}
{{{highlight(r=(9+2u)/u)}}} This is still one equation in TWO unknown variables.


This will have infinite solutions but there is a restriction.  
{{{highlight(0<u<3)}}}  is that restriction.  You may compute r accordingly for its RANGE.