:①(38.7－18.6÷3.1)×0.8 =26.16
②7.09×10.8－8×0.709=0.709×108-8×0.709=0.709×（108-8）=70.9
③15.6×13.1－15.6－15.6×2.1=15.6×（13.1-1-2.1）=156
④20÷1.25÷8=20÷（1.25×8）=2лл