I would guess that x >= 100 is redundant.
Try x >= (x+y)/3
Which leads to 2x - y >= 0
Bob's books ships 300-450 books a month. Some are shipped from store A and some are shipped from store B, but at least one third must be shipped from each warehouse. It costs 37 cents from store A and 55 cents from store B. Minimize the cost.
So x=number from store A and y=number from store B.
Now would the constraints be:
x>=100, y>=100, x<=300, x<=300
or would they be
x>=(y/3), x<=2y/3, y>=(x/3), y<=(2x/3)