Question 1010909
Let {{{ a }}} = gallons of milk with 4% butterfat
Let {{{ b }}} = gallons of cream with 40% butterfat
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(1) {{{ a + b = 36 }}}
(2) {{{ ( .04a + .4b ) / 36 = .2 }}}
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(2) {{{ .04a + .4b = .2*36 }}}
(2) {{{ 4a + 40b = 720 }}}
Multiply both sides of (1) by {{{ 4 }}} and 
subtract (1) from (2)
(2) {{{ 4a + 40b = 720 }}}
(1) {{{ - 4a - 4b = -144 }}}
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{{{ 36b = 576 }}}
{{{ b = 16 }}}
and
(1) {{{ a + b = 36 }}}
(1) {{{ a = 20 }}}
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20 gallons of milk with 4% butterfat
16 gallons of cream with 40% butterfat
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check:
(2) {{{ ( .04a + .4b ) / 36 = .2 }}}
(2) {{{ ( .04*20 + .4*16 ) / 36 = .2 }}}
(2) {{{ ( .8 + 6.4 ) / 36 = .2 }}}
(2) {{{ 7.2 = .2*36 }}}
(2) {{{ 7.2 = 7.2 }}}
OK