Question 545063
Let {{{s}}} = number of student tickets sold
Let {{{n}}} = number of non-student tickets sold
given:
(1) {{{ s + n = 2000 }}}
(2) {{{ 9s + 11n = 20960 }}}
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Multiply both sides of (1) by {{{9}}} and 
subtract (1) from (2)
(2) {{{ 9s + 11n = 20960 }}}
(1) {{{ -9s - 9n = -18000 }}}
{{{ 2n = 2960 }}}
{{{ n = 1480 }}}
and, since
(1) {{{ s + n = 2000 }}}
(1) {{{ s + 1480 = 2000 }}}
(1) {{{ s = 520 }}}
520 student tickets were sold
check answer:
(2) {{{ 9*520 + 11*1480 = 20960 }}}
(2) {{{ 4680 + 16280 = 20960 }}}
(2) {{{ 20960 = 20960 }}}
OK