Every pair that multiplies to your number — and why you only test up to the square root.
Number
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A factor tree that grows branch by branch, with the reason each prime was chosen.
The ladder method — divide both numbers together until nothing is left in common.
Same ladder as the GCF, but you multiply in an L shape — down the side and along the bottom.
Test one number against every rule at once, and see why each one works.