Question 553486
Let {{{ s }}} = the speed of the boat in still water
Let {{{ c }}} = the rate of the current
{{{ s + c }}} = the speed of the boat going downstream
{{{ s - c }}} = the speed of the boat going upstream
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given:
Going downstream:
(1) {{{ 120 = ( s + c )*3 }}}
Going upstream:
(2) {{{ 120 = ( s - c )*4 }}}
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(1) {{{ 3s + 3c = 120 }}}
(2) {{{ 4s - 4c = 120 }}}
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(1) {{{ s + c = 40 }}}
(2) {{{ s - c = 30 }}}
Add the equations
{{{ 2s = 70 }}}
{{{ s = 35 }}}
and
(1) {{{ s + c = 40 }}}
(1) {{{ 35 + c = 40 }}}
(1) {{{ c = 5 }}}
The speed of the current is 5 mi/hr