Which of the following number sentences is an example of the associative property of multiplication?
1. (12 × 3) × 8 = 12 (3 × 8)
2. 6 × 1 = 6
3. 4 × 5 = 5 × 4
4. 20 × (10 + 6) = (20 × 10) + (20 × 6)