Question 689006
Let {{{ s }}} = the speed of the boat in still water
Let {{{ c }}} = the speed of the current
given:
(1) {{{ s + c = 70/5 }}}
(2) {{{ s - c = 70/7 }}}
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Add the equations
(1) {{{ s + c = 70/5 }}}
(2) {{{ s - c = 70/7 }}}
{{{ 2s = 70/5 + 70/7 }}}
{{{ 2s = ( 7*70 ) / 35 + ( 5*70 ) / 35 }}}
{{{ 2s = 490 / 35 + 350 / 35 }}}
{{{ 2s = 840 / 35 }}}
{{{ s = 420 / 35 }}}
{{{ s = 12 }}}
The speed of the boat in still water is 12 km/hr