Let the two original numbers be a (unchanged) and b (changed).<br><br>&bull; Original sum: a + b = 9 × 2 = 18<br>&bull; After change: a + 8 = 10 × 2 = 20<br>&bull; Unchanged number a = 20 − 8 = 12<br><br>&bull; Check: Original other number b = 18 − 12 = 6<br>&bull; Change 6 to 8: (12 + 8) ÷ 2 = 20 ÷ 2 = 10. ✓<br><br>Final Answer: 12