Question 1130045
{{{butterfat/milk=0.04}}}


x liters of 7% butterfat
20-x liters of 3% butterfat


{{{(0.07x+0.03(20-x))/20=0.04}}}

.

.

{{{x=5}}}


15 liters of the 3% butterfat milk
5 liters of the 7% butterfat milk