Question 1114579
Speed of current c
Speed of boat if still water, r

t, time being same in both directions


{{{system((r+c)=60/t,(r-c)=30/t)}}}


GIVEN also c=6.
{{{system(r+6=60/t,r-6=30/t)}}}

{{{t/60=1/(r+6)}}}
{{{t=60/(r+6)}}}

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{{{t/30=1/(r-6)}}}
{{{t=30/(r-6)}}}

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Solve this equation:
{{{highlight_green(60/(r+6)=30/(r-6))}}}

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{{{2/(r+6)=1/(r-6)}}}

{{{r+6=2(r-6)}}}

{{{r+6=2r-12}}}

{{{highlight(18=r)}}}