Cute idea... good way to get them to learn square roots without them knowing it because they know the numbers under the post its! It is very inexpensive and not at all time consuming to make. The only problem I can think of with this would be the postit notes loosing their ability to stick onto the clock's face.
Cubed Root: Find the prime factors of the number under the radical; look for groups of 3 of the same number, like 3 - 2's (cubed root of 8) to take out of the radical; if negative, can factor out the -1
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