4538+3998
=4538+4000-2
=8538-2
=8536

3.2×10.1
=3.2×10+3.2×0.1
=32+0.32
=32.32

38×99+38
=38×（1+99）
=38×100
=3800
7/15÷5+8/15×1/5
=（7/15+8/15）×1/5
=1/5

1-0.225÷0.9-0.75
1-（0.25+0.75）
=1-1
=0

65×65/66
=65×（1-1/66）
=65-65/66
=64又1/66
2.5×3.2×12.5
=2.5×4×（0.8×12.5）
=10×10
=100