Question 921813
Let {{{ s }}} = the speed rowing in still water in mi/hr
{{{ s - 2 }}} = his speed rowing upstream in mi/hr
{{{ s + 2 }}} = his speed rowing downstream in mi/hr
Let {{{ t }}} = time in hrs to row upstream
{{{ t - 2 }}} = time in hrs to row downstream
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Equation for rowing upstream:
(1) {{{ 13 = ( s-2 )*t }}}
Equation for rowing downstream:
(2) {{{ 13 = ( s+2 )*( t-2 ) }}}
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(2) {{{ 13 = t*( s+2 ) - 2s - 4 }}}
and
(1) {{{ t = 13 / ( s-2 ) }}}
Substitute (1) into (2)
(2) {{{ 17 = ( 13/( s-2) )*( s+2 ) - 2s  }}}
(2) {{{ 17*( s-2 ) = 13*( s+2 ) - 2s*( s-2 ) }}}
(2) {{{ 17s - 34 = 13s + 26 - 2s^2 + 4s }}}
(2) {{{ 2s^2 + 17s - 13s - 4s = 34 + 26 }}}
(2) {{{ 2s^2 = 60 }}}
(2) {{{ s^2 = 30 }}}
(2) {{{ s = 5.477 }}}
John's rowing speed is 5.477 mi/hr
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check:
(1) {{{ 13 = ( s-2 )*t }}}
(1) {{{ 13 = ( 5.477-2 )*t }}} 
(1) {{{ 13 = 3.477t }}}
(1) {{{ t = 3.739 }}}
and
(2) {{{ 13 = ( s+2 )*( t-2 ) }}}
(2) {{{ 13 = ( 5.477+2 )*( 3.739-2 ) }}}
(2) {{{ 13 = 7.477*1.739 }}}
(2) {{{ 13 = 13.003 }}}
Error due to rounding off, I think
Check all my math