is constructed by making arcs centered at A and B without changing the compass width. Which equation is not necessarily true?

The answer is B.   Because there are 8 ribbons in all. 1 ribbon is black. so the equation would be 1/8. Assuming you don't draw the black ribbon on the first try, then you would try again. The second time you draw a ribbon you don't put the ribbon you drew back in. That means there are only 7 ribbons the second time. The 1 black ribbon is still in the bag than the equation would be 1/7. Then you add the two together. So it would be 1/8 + 1/7 = 1/15. Remember to only add the bottom numbers. not the top ones unless they are different colors or objects.