Question 843383
Let {{{ s }}} = her speed in still water in km/hr
Let {{{ c }}} = the speed of the current in km/hr
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Equation for rowing downstream:
(1) {{{ 11 = ( s + c )*2 }}}
Equation for rowing upstream:
(2) {{{ 11 = ( s - c )*3 }}}
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This is 2 equations and 2 unknowns 
so it's solvable
(1) {{{ 11 = 2s + 2c }}}
and
(2) {{{ 11 = 3s - 3c }}}
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Multiply both sides of (1) by {{{3}}} and
both sides of (2) by {{{2}}}
Then add the equations
(1) {{{ 6s + 6c = 33 }}}
(2) {{{ 6s - 6c = 22 }}}
{{{ 12s = 55 }}}
{{{ s = 4.5833 }}}
She can row 4.5833 km/hr in still water
check:
(1) {{{ 2s + 2c = 11 }}}
(1) {{{ 2*(55/12) + 2c = 11 }}}
(1) {{{ 55/6 + 2c = 11 }}}
(1) {{{ 2c = 66/6 - 55/6 }}}
(1) {{{ 2c = 11/6 }}}
(1) {{{ c = 11/12 }}}
(1) {{{ c = .9167 }}}
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(2) {{{ 3s - 3c = 11 }}}
(2) {{{ 3*(55/12) - 3*(11/12) = 11 }}}
(2) {{{ 3*55 - 3*11 = 11*12 }}}
(2) {{{ 3*5 - 3 = 12 }}}
(2) {{{ 15 - 3 = 12 }}}
(2) {{{ 12 = 12 }}}
OK
(1) {{{ 2s + 2c = 11 }}}
(1) {{{ 2*(55/12) + 2*(11/12) = 11 }}}
(1) {{{ 2*55 + 2*11 = 11*12 }}}
(1) {{{ 2*5 + 2 = 12 }}}
(1) {{{ 10 + 2  = 12 }}}
(1) {{{ 12 = 12 }}}
OK