Question 1149432
Let {{{ a }}} = liters of 3% milk needed
Let {{{ b }}} = liters of 84% milk needed
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(1) {{{ a + b = 135 }}} liters
(2) {{{ ( .03a + .84b ) / 135 = .78 }}}
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(2) {{{  3a + 84b = 78*135 }}}
(2) {{{  3a + 84b = 10530 }}}
(2) {{{ a + 28b = 3510 }}}
Subtract (1) from (2)
(2) {{{ a + 28b = 3510 }}}
(1) {{{ -a - b = -135 }}}
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{{{ 27b = 3375 }}}
{{{ b = 125 }}}
and
(1) {{{ a + 125 = 135 }}}
(1) {{{ a = 10 }}}
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10 liters of 3% milk are needed
125  liters of 84% milk are needed
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check:
(2) {{{ ( .03*10 + .84*125 ) / 135 = .78 }}}
(2) {{{ ( .3 + 105 ) / 135 = .78 }}}
(2) {{{ 105.3 / 135 = .78 }}}
(2) {{{ .78 = .78 }}}
OK