Question 412152
Let {{{a}}} = quarts of 6% milk needed
Let {{{b}}} = quarts of 3% milk needed
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In words:
(total quarts of butterfat)/(total quarts of milk) = 5%
(1) {{{(.06a + .03b)/(a + b) = .05}}}
(2) {{{a + b = 45}}}
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From (1)
(1) {{{.06a + .03b = .05*(a + b)}}}
(1) {{{.06a + .03b = .05a + .05b}}}
(1) {{{.01a = .02b}}}
(1) {{{a = 2b}}}
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(2) {{{2b + b = 45}}}
(2) {{{3b = 45}}}
(2) {{{b = 15}}}
and, 
(2) {{{a = 45 - 15}}}
(2) {{{a = 30}}}
30 quarts of 6% and 15 quarts of 3% butterfat milk are needed
check answer:
(1) {{{.06a + .03b = .05*(a + b)}}}
(1) {{{.06*30 + .03*15 = .05*45}}}
(1) {{{1.8 + .45 = 2.25}}}
(1) {{{2.25 = 2.25}}}
OK