Question 585870
Let {{{a}}} = number of white milks sold
Let {{{b}}} = number of chocolate milks sold
given:
(1) {{{ a + b = 50 }}}
(2) {{{ 25a + 30b = 1330 }}}  ( in cents )
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Multiply both sides of (1) by {{{ 25 }}}
and subtract (1) from (2)
(2) {{{ 25a + 30b = 1330 }}}
(1) {{{ -25a - 25b = -1250 }}}
{{{ 5b = 80 }}}
{{{ b = 16 }}}
and, since
(1) {{{ a + b = 50 }}}
(1) {{{ a + 16 = 50 }}}
(1) {{{ a = 34 }}}
34 white milks were sold
16 chocolate milks were sold
check answer:
(2) {{{ 25a + 30b = 1330 }}} 
(2) {{{ 25*34 + 30*16 = 1330 }}
(2) {{{ 850 + 480 = 1330 }}}
(2) {{{ 1330 = 1330 }}}
OK