Question 777738
Let {{{ a }}} = liters of 1% milk needed
Let {{{ b }}} = liters of 4% milk needed
{{{ .01a }}} = liters of fat in 1% milk
{{{ .04b }}} = liters of fat in 4% milk
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(1) {{{ a + b = 60 }}}
(2) {{{ ( .01a + .04b ) / 60 = .02 }}}
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(2) {{{ .01a + .04b = .02*60 }}}
(2) {{{ .01a + .04b = 1.2 }}}
(2) {{{ a + 4b = 120 }}}
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Subtract (1) from (2) 
(2) {{{ a + 4b = 120 }}}
(1) {{{ -a - b = -60 }}}
{{{ 3b = 60 }}}
{{{ b = 20 }}}
and, since
(1) {{{ a + b = 60 }}}
(1) {{{ a = 60 - 20 }}}
(1) {{{ a= 40 }}}
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40 liters of 1% milk are needed
20 liters of 4% milk are needed
check:
(2) {{{ ( .01a + .04b ) / 60 = .02 }}}
(2) {{{ ( .01*40 + .04*20 ) / 60 = .02 }}}
(2) {{{ ( .4 + .8 ) / 60 = .02 }}}
(2) {{{ 1.2 = .02*60 }}}
(2) {{{ 1.2 = 1.2 }}}
OK