Question 503947
Let {{{p}}} = boxes of pencils he bought
Let {{{e}}} = boxes of erasers he bought
given:
(1) {{{ 3p + 1.5e = 30 }}}
(1) {{{ 30p + 15e = 300 }}}
and
(2) {{{ e = p + 2 }}}
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Substitute (2) into (1)
(1) {{{ 30p + 15*( p + 2 ) = 300 }}}
(1) {{{ 30p + 15p + 30 = 300 }}}
(1) {{{ 45p = 270 }}}
(1) {{{ p = 6 }}}
He bought 6 boxes of pencils
check:
(1) {{{ 3p + 1.5e = 30 }}}
(1) {{{ 3*6 + 1.5e = 30 }}}
(1) {{{ 1.5e = 30 - 18 }}}
(1) {{{ 1.5e = 12 }}}
(1) {{{ e = 8 }}}
and
(2) {{{ e = p + 2 }}}
(2) {{{ 8  = 6 + 2 }}}
OK