Question 609323
Let {{{ a }}} = units of whole wheat flour needed
Let {{{ b }}} = units of whole milk needed
given:
{{{ a }}} units of whole wheat flour has {{{ 13.6a }}} grams protein
{{{ a }}} units of whole wheat flour has {{{ 2.5a }}} grams fat
{{{ b }}} units of whole milk has {{{ 3.4b }}} grams protein
{{{ b }}} units of whole milk has {{{ 3.7b }}} grams fat
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(1) {{{ 13.6a + 3.4b = 75 }}}
(2) {{{ 2.5a + 3.7b = 15 }}}
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(1)  {{{ 136a + 34b = 750 }}}
(2)  {{{ 25a + 37b = 150 }}}
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Multiply both sides of (2) by {{{ 34/37 }}}
(2) {{{ 25*(34/37)a + 34b = (34/37)*150 }}}
(2) {{{ 22.973a + 34b = 137.838 }}}
Subtract (2) from (1)
(1) {{{ 136a + 34b = 750 }}}
(2) {{{ -22.973a - 34b = -137.838 }}}
{{{ 113.027a = 612.162 }}}
{{{ a = 5.416 }}} units
and, since
(2) {{{ 25a + 37b = 150 }}}
(2) {{{ 25*5.416 + 37b = 150 }}}
(2) {{{ 135.4 + 37b = 150 }}}
(2) {{{ 37b = 150 - 135.4 }}}
(2) {{{ 37b = 14.6 }}}
(2) {{{ b = .395 }}}
5.4 units of whole wheat flour is needed
.4 units of whole milk are needed
check:
(1) {{{ 13.6*5.4 + 3.4*.4 = 75 }}}
(1) {{{ 73.44 + 1.36 = 75 }}}
(1) {{{ 74.8 = 75 }}} ( close )
(2) {{{ 2.5*5.4 + 3.7*.4 = 15 }}}
(2) {{{ 13.5 + 1.48 = 15 }}}
(2) {{{ 14.98 = 15 }}} 
OK