①5.1÷5+4.9×20%
=5.1×0.2+4.9×0.2
=(5.1+4.9)×0.2
=10×0.2
=2
②（8/9+1/6）×72%
=8/9×0.72+1/6×0.72
=0.64+0.12
=0.76
③3/4÷（75%+5/8）
=3/4÷（3/4+5/8）
=3/4÷11/8
=3/4×8/11
=6/11
④125%×7.6-1.25×3.1+10/8×5.5
=1.25×（7.6-3.1+5.5）
=1.25×10
=12.5简算哦！都简算了哦