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'Fractions in a Box' printed from https://nrich.maths.org/
Remember the booklet in the corner of the box is square, so how
many holes could it take
up?
The whole box, without the space for the booklet, would fit $100$
discs, so the number of discs must be $100$, less the square number
taken up by the booklet.
What do you know about the factors of the total number of
discs?