Answer:
13 sticks
Explanation:
100/12=8 1/3
Tells us he needs to follow the recipe 8 times and third the ingredients the 9th time
8 1/3*1 1/2
25/3*3/2
75/6
Needs 12.5 Sticks of butter
Must buy 13 sticks of butter since you can't buy less or half a stick