Let the two original numbers be a (unchanged) and b (replaced).<br><br>&bull; Original sum: a + b = 25 × 2 = 50<br>&bull; After replacement: a + 21 = 22.5 × 2 = 45<br>&bull; Unchanged number a = 45 − 21 = 24<br><br>&bull; Check: Original other number b = 50 − 24 = 26<br>&bull; Replace 26 with 21: (24 + 21) ÷ 2 = 22.5. ✓<br><br>Final Answer: 24