Assuming the original percentages are correct (i.e., three of the cow's grandparents were registered fullblood Angus and one of her grandparents was a registered fullblood Hereford ... and none of those animals was upgraded from some other breed) ...
87.5% Angus
12.5% Hereford
Each breed has their own rules about calculating percentages when an animal is the result of an upgrading program (i.e., taking a commercial, unpedigreed herd and converting them (over a few generations) to pedigreed animals via using other pedigreed animals).