Concept: The question tests on basics of of Mixtures/Average

Solution:

Total 10 gallons

Let x gallons of milk be used for the 2% Butterfat. Then 10 - x gallons will be for the 3.5% butterfat

Average Concentration = C1Q1+C2Q2/Total Quantity
 3=2x+3.5(10−x)/10

3=2x+3.5(10−x)10

30 = 2x + 35 - 3.5x

1.5x = 5

x = 50/15 = 10/3

Option B