Percentage is defined as parts of an amount in 100 parts of a basis. This must be stated mathamatically for better understanding and for calculations. It is the ratio:
% = (parts / base) × 100.
In this case you need to express the amount of butter as a part in 100 parts of all purpose flour, which, using the above definition, is:
% butter = (0.75 pounds of butter / 2 pound of flour) × 100