From the previous working, the unchanged number is 12.<br><br>&bull; Original sum a + b = 9 × 2 = 18<br>&bull; a = 12, so b = 18 − 12 = 6<br>&bull; The number that was replaced by 8 is 6.<br><br>&bull; Check: Replace 6 with 8: (12 + 8) ÷ 2 = 20 ÷ 2 = 10. ✓<br><br>Final Answer: 6