Question 780612
Let {{{ s }}} = the speed of the boat in km/hr in still water
Let {{{ d }}} = the one-way distance the boat travels in km
Going upstream:
{{{ d = ( s - 5 )*(1/2) }}}
Going downstream:
{{{ d = ( s + 5)*(10/60) }}}
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By substitution:
{{{ ( s - 5 )*(1/2)  =  ( s + 5)*(1/6) }}}
Multiply both sides by {{{ 6 }}}
{{{ 3*( s - 5 ) = s + 5 }}}
{{{ 3s - 15 = s + 5 }}}
{{{ 2s = 20 }}}
{{{ s = 10 }}}
The speed of the boat in still water is 10 km/hr
check:
{{{ d = ( s - 5 )*(1/2) }}}
{{{ d = ( 10 - 5 )*(1/2) }}}
{{{ d = 5/2 }}} km
and
{{{ d = ( s + 5)*(1/6) }}}
{{{ d = ( 10 + 5 )*(1/6) }}}
{{{ d = 15/6 }}}
{{{ d = 5/2 }}} km
OK