7.2×10.2
=7.2×10+7.2×0.2
=72+1.44
=73.44
13又2/13 - 2.25 - 7又3/4
=13又2/13 - (2.25 + 7又3/4)
=13又2/13 - 10
=3又2/13
（5又5/9 - 0.8 + 2又4/9） × 12.5
=[(5又5/9 + 2又4/9) - 0.8]× 12.5
=(8-0.8)× 12.5
=8 × 12.5 - 0.8 × 12.5
=100-10
=90
2.25 × 7又1/5 + 2.8 除以 4/9
=2.25×7.2+2.8×(9/4)
=2.25×7.2+2.8×2.25
=2.25×(7.2+2.8)
=2.25×10
=22.5