Question 1092529
Let {{{ s }}} = the speed of the boat in still water
Let {{{ t }}} = time in hrs to go either 
downstream or upstream
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Equation for going downstream:
(1) {{{ 210 = ( s + 4 )*t }}}
Equation for going upstream:
(2) {{{ 180 = ( s - 4 )*t }}}
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(1) {{{ t = 210 / ( s + 4 ) }}}
Plug (1) into (2)
(2) {{{ 180 = ( s - 4 )*210 / ( s + 4 ) }}}
(2) {{{ 180*( s + 4 ) = 210*( s - 4 ) }}}
(2) {{{ 180s + 720 = 210s - 840 }}}
(2) {{{ 30s = 1560 }}}
(2) {{{ s = 52 }}}
The speed in still water is 52 mi/hr
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check:
(1) {{{ t = 210 / ( 52 + 4 ) }}}
(1) {{{ t = 210 / 56 }}}
(1) {{{ t = 3.75 }}} hrs
and
(2) {{{ t = 180 / ( 52 - 4 ) }}}
(2) {{{ t = 180 / 48 }}}
(2) {{{ t = 3.75 }}} hrs
OK