Question 539596
{{{ a }}} = number of students attending
{{{ b }}} = number of adults attending
given:
(1) {{{ a + b = 155 }}}
(2) {{{ 2.5a + 4b = 492.5 }}}
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(2) {{{ 25a + 40b = 4925 }}}
(2) {{{ 5a + 8b = 985 }}}
Multiply both sides of (1) by {{{5}}}
and subtract (1) from (2)
(2) {{{ 5a + 8b = 985 }}}
(1) {{{ -5a -5b = -775 }}}
{{{ 3b = 210 }}}
{{{ b = 70 }}}
and, since
(1) {{{ a + 70 = 155 }}}
(1) {{{ a = 85 }}}
85 students attended
70 adults attended
(2) {{{ 5*85 + 8*70 = 985 }}}
(2) {{{ 425  + 560 = 985 }}}
(2) {{{ 985 = 985 }}}
OK