Arrange equal rows without leftovers. Save each factor pair once. When a factor tree appears, split its branches until every leaf is prime.

For 12, find 1 × 12, 2 × 6 and 3 × 4. A factor tree can split 12 into 3 × 4, then 4 into 2 × 2: 12 = 2 × 2 × 3. A prime number has exactly two positive factors: 1 and itself. One is not prime.

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