Question 937832
Let {{{ d }}} = the one-way distance ( up or down river )
Let {{{ t }}} = time in hrs to go upriver
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Convert {{{ 56 }}} min to hours
{{{ 56/60 = .9333 }}} hrs
{{{ 2.9333 - t }}} = time in hrs to go downriver
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Equation for going upstream:
(1) {{{ d = 17.5t }}}
Equation for going downstream:
(2) {{{ d = 21*( 2.9333 - t ) }}}
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Substitute (1) into  (2)
(2) {{{ 17.5t = 21*( 2.9333 - t ) }}}
(2) {{{ 17.5t = 61.6 - 21t }}}
(2) {{{ 38.5t = 61.6 }}}
(2) {{{ t = 1.6 }}} hrs
Plug this result back into (1)
(1) {{{ d = 17.5*1.6 }}}
(1) {{{ d = 28 }}}
The boat went 28 mi upriver
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check:
(2) {{{ d = 21*( 2.9333 - t ) }}}
(2) {{{ 28 = 21*( 2.9333 - 1.6 ) }}}
(2) {{{ 28 = 61.6 - 33.6 }}}
(2) {{{ 28 = 28 }}}
OK