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(Biological permutations)
A DNA sequence has 4 bases: A, T, C, G. How many sequences of length 10 exist? 4^10 = 1,048,576.
If you want sequences with exactly 3 A's and 7 T's (order matters), count = 10! / (3! × 7!) = 120.
Permutations model genetic variation and sequence space.
