Question 952059
The current, {{{ c = 2 }}} m/sec
Swimming speed is {{{ s = 3 }}} m/sec
{{{ 3 + 2 = 5 }}} m/sec downstream
{{{ 3 - 2 = 1 }}} m/sec upstream
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Let {{{ d }}} = one-way distance he swims
Let {{{ t }}} = his time in minutes to go downstream
Convert {{{ 4 }}} min to sec
{{{ 4*60 = 240 }}} sec
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Equation for downstream:
(1) {{{ d = 5*t }}}
Equation for upstream:
(2) {{{ d = 1*( 240 - t ) }}}
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Substitute (1) into (2)
(2) {{{ 5t = 240 - t }}}
(2) {{{ 6t = 240 }}}
(2) {{{ t = 40 }}}
and
(1) {{{ d = 5*t }}}
(1) {{{ d = 5*40 }}}
(1) {{{ d = 200 }}}
He swam 200 m
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check:
(2) {{{ d = 1*( 240 - t ) }}}
(2) {{{ 200 = 1*( 240 - 40 ) }}} 
(2) {{{ 200 = 200 }}}
OK