Question 601764
Let {{{ a }}} = gallons of skim milk needed
Let {{{ b }}} = gallons of 3.6% butterfat
{{{ .02*225 = 4.5 }}} gallons of butterfat in 2%  milk
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(1) {{{ a + b = 225 }}}
(2) {{{ ( .036b ) / 225 = .02 }}}
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(2) {{{ .036b = .02*225 }}}
(2) {{{ .036b = 4.5 }}}
(2) {{{ b = 125 }}}
and, since
(1) {{{ a + b = 225 }}}
(1) {{{ a + 125 = 225 }}}
(1) {{{ a = 100 }}}
100 gallons of skim milk is needed
125 gallons of 3.6% butterfat is needed