<PRE><B>Question 718767</B>
Let {{{ a }}} = number of adults who attended
Let {{{ c }}} = number of children who attended
given:
(1) {{{ a + c = 900 }}}
(2) {{{ 7a + 4c = 5100 }}}
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Multiply both sides of (1) by {{{ 4 }}}
and subtract (1) from (2)
(2) {{{ 7a + 4c = 5100 }}}
(1) {{{ -4a - 4c = -3600 }}}
{{{ 3a = 1500 }}}
{{{ a = 500 }}}
and, since
(1) {{{ 500 + c = 900 }}}
(1) {{{ c = 400 }}}
500 adults attended
check:
(2) {{{ 7a + 4c = 5100 }}}
(2) {{{ 7*500 + 4*400 = 5100 }}}
(2) {{{ 3500 + 1600 = 5100 }}}
(2) {{{ 5100 = 5100 }}}
OK
</PRE>