Question 402406

{{{b + c}}} = rate going downstream
{{{b - c}}} = rate going upstream
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given:
upstream:
(1) {{{60 = (b - c)*5}}} km
downstream:
(2) {{{60 = (b + c)*3}}} km
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This is 2 equations and 2 unknowns
so it's solvable
(1) {{{5b - 5c = 60}}}
(2) {{{3b + 3c = 60}}}
Multiply both sides of (1) by {{{3}}}
and both sides of (2) by {{{5}}}
Then add the equations
(1) {{{15b - 15c = 180}}}
(2) {{{15b + 15c = 300}}}
{{{30b = 480}}}
{{{b = 16}}}
Plug this back into (1)
(1) {{{5*16 - 5c = 60}}}
{{{80 - 5c = 60}}}
{{{5c = 20}}}
{{{c = 4}}}
16 km/hr is the rate of the boat in still water
4 km/hr is the rate of the current
check answer:
(1) {{{60 = (16 - 4)*5}}} km
{{{60 = 12*5}}}
{{{60 = 60}}}
(2) {{{60 = (16 + 4)*3}}} km
{{{60 = 20*3}}}
{{{60 = 60}}}
OK