You need to use 7.5 cups. The butter is 6 times the original amount (1/2 stick). So the flour also needs to be 6 times the original amount.
Butter
3/.5 =6
Flour
__/1.25 =6
You can rearrange the flour equation to get
1.25*6 = ___ the blank is 7.5 cups. The total amount of flour needed is 7.5 cups
