Odd non-primes: 0 → 0
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- Non-primes: each has 3 choices → $ 3 \times 3 = 9 $, but we must restrict to even non-primes? No: for sum parity, only the oddness matters.
- For Case 1:
- Odd primes: 0 → contribution: 0 odd values
- Even non-primes: 0 → 0
- So total odd count = 0 → even → valid
- Choose 2 positions for primes: $ \binom{4}{2} = 6 $
