Question 1057326
Let {{{ s }}} = the speed of her boat in still water
Let {{{ c }}} = the speed of the current 
{{{ s + c }}} = her speed going downstream
{{{ s - c }}} = her speed going upstream
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Equation for going downstream:
(1) {{{ 21 = ( s + c )*3 }}}
Equation for going upstream:
(2) {{{ 21 = ( s - c )*3.5 }}}
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Add the equations:
(1) {{{ s + c = 7 }}}
(2) {{{ s - c = 6 }}}
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{{{ 2s = 13 }}}
{{{ s = 6.5 }}}
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the speed of her boat in still water is 6.5 mi/hr
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check answer:
(1) {{{ s + c = 7 }}}
(1) {{{ 6.5 + c = 7 }}}
(1) {{{ c = .5 }}} mi/hr
and
(2) {{{ s - c = 6 }}}
(2) {{{ 6.5 - c = 6 }}}
(2) {{{ c = .5 }}}
OK