Question 518241
Let {{{ a }}} = number of 39c stamps
Let {{{ b }}} = number of 24c stamps
given:
(1) {{{ a + b = 40 }}}
(2) {{{ 39a + 24b = 1485 }}} ( in cents )
Multiply both sides of (1) by {{{24}}}
and subtract (1) from (2)
(2) {{{ 39a + 24b = 1485 }}}
(1) {{{ -24a - 24b = -960 }}}
{{{ 15a = 525 }}}
{{{ a = 35 }}}
and, since
(1) {{{ a + b = 40 }}}
(1) {{{ b = 40 - 35 }}}
(1) {{{ b = 5 }}}
They buy 35 39c stamps
They buy 5 24c stamps
check:
(2) {{{ 39*35 + 24*5 = 1485 }}}
(2) {{{ 1365 + 120 = 1485 }}}
(2) {{{ 1485 = 1485 }}}
OK