Question 841990
Let {{{ s }}} = the boat's speed in still water
Let {{{ c }}} = the speed of the current
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Equation for going downstream:
(1) {{{ 315 = ( s + c )*15 }}}
Equation for going upstream:
(2) {{{ 315 = ( s - c )*35 }}}
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(1) {{{ s + c = 315/15 }}}
(1) {{{ s + c = 21 }}}
and
(2) {{{ s - c = 315/35 }}}
(2) {{{ s - c = 9 }}}
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Add (1) and (2)
(1) {{{ s + c = 21 }}}
(2) {{{ s - c = 9 }}}
{{{ 2s = 30 }}}
{{{ s = 15 }}}
and
(2) {{{ s - c = 9 }}}
(2) {{{ 15 - c = 9 }}}
(2) {{{ c = 15 - 9 }}}
(2) {{{ c = 6 }}}
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The boat's speed in still water is 15 mi/hr
The speed of the current is 6 mi/hr