Having flogged the question to death mathematically, we can have even more fun by considering the practical matters that add another layer of ill-posedness:
Perhaps the blanket is rectangular has selvedge along one side (intended to be the bottom) and a border is needed only on three sides, or selvedge along two sides (used as the long sides) and a border is needed on only two.
Perhaps the border material is crocheting cotton, in which case the correct answer depends both on the perimeter of the blanket and the crocheting pattern used (and will be much larger than the perimeter in linear units).
You’ve actually got the point.
The problem isn’t one of math. It is training to look to the authority figure’s intent as to context and narrative in order to regurgitate the approved answer.