Question 699611
Let {{{ a }}} = number of 30 cent stamps she bought
Let {{{ b }}} = number of 20 cent stamps she bought
given:
(1) {{{ 30a + 20b = 360 }}} ( in cents )
(2) {{{ b = a - 2 }}}
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Substitute (2) into (1)
(1) {{{ 30a + 20*( a - 2 ) = 360 }}} 
(1) {{{ 30a + 20a - 40 = 360 }}}
(1) {{{ 50a = 400 }}}
(1) {{{ a = 8 }}}
and, since
(2) {{{ b = a - 2 }}}
(2) {{{ b = 8 - 2 }}}
(2) {{{ b = 6 }}}
She bought 8 30 cent stamps and 6 20 cent stamps
check:
(1) {{{ 30*8 + 20*6 = 360 }}} 
(1) {{{ 240 + 120 = 360 }}}
(1) {{{ 360 = 360 }}}
OK