Question 1036893
Let {{{ p }}} = number of boxes of pens sold
Let {{{ b }}} = number of batteries sold
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(1) {{{ 8p + 2b = 936 }}}
(2) {{{ b = 8p }}}
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Substitute (2) into (1)
(1) {{{ 8p + 2*(8p) = 936 }}}
(1) {{{ 8p + 16p = 936 }}}
(1) {{{ 24p = 936 }}}
(1) {{{ p = 39 }}}
and 
(2) {{{ b = 8p }}}
(2) {{{ b = 8*39 }}} 
(2) {{{ b = 312 }}}
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39 boxes of pens were sold
312 batteries were sold
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check:
(1) {{{ 8p + 2b = 936 }}}
(1) {{{ 8*39 + 2*312 = 936 }}}
(1) {{{ 312 + 624 = 936 }}}
(1) {{{ 936 = 936 }}}
OK