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