Question 1019724
Convert gallons to quarts:
{{{ 2*4 = 8 }}} quarts
Let {{{ x }}} = quarts of milk should be 
drained and replaced by pure butterfat
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{{{ .02x }}} = pure butter fat in {{{ x }}}
quarts of milk
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{{{ .02*8 = .16 }}} = original quarts of butterfat
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Note that you start with 8 quarts and
end up with 8 quarts
{{{ ( .16 - .02x + x ) / 8 = .05
{{{ ( .16 + .98x ) / 8 = .05 }}}
{{{ .16 + .98x = .4 }}}
{{{ .98x = .24 }}}
{{{ x = .2449 }}} 
.2449 quarts of milk must be drained and
replaced with pure butterfat
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check:
{{{ .02*.2449 = .004898 }}} quarts of
butterfat is drained off
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{{{ .2449 }}} quarts of butterfat is added
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{{{ ( .16 - .004898 + .2449 ) / 8 = .05 }}}
{{{ .4/8 = .05 }}}
{{{ .4 = .4 }}}
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Strange numbers -definitely get a 2nd opinion
-unless the answer is already known.