Practice: compute 63.5 − 28.7 mentally.

34.8 — balance the subtraction to 64.8 − 30.0.

Add 1.3 to both numbers so the subtrahend becomes round:

63.5 − 28.7 = 64.8 − 30.0 = 34.8.

Balancing is exact because adding the same amount to both numbers preserves the difference.