Question 490068
Let {{{r}}} = number of red ribbons she sold
Let {{{b}}} = number of blue ribbons she sold
given:
(1) {{{ r + b = 36 }}}
(2) {{{ r = b + 6 }}}
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You can solve for {{{r}}} and {{{b}}} now
Subtract {{{b}}} from both sides of (2),
then add the equations
(1) {{{ r + b = 36 }}}
(2) {{{ r - b =  6 }}}
{{{ 2r = 42 }}}
{{{ r = 21 }}}
and, since
(1) {{{ r + b = 36 }}}
(1) {{{ 21 + b = 36 }}}
(1) {{{ b = 15 }}}
She sold 21 red ribbons and 15 blue ribbons