69.8-(4.7+19.8)=(69.8-19.8)-4.7=50-4.7=45.3
4.74-8.98+15.98=4.74+(15.98-8.98)=4.74+7=11.74
2.5-1.98=2.5-(2-0.02)=2.5-2+0.02=0.52
7.2*9.9=7.2*(10-0.1)=72-0.72=71.28
7.2*10.1=7.2(10.0.1)=72+0.72=72.72
88.8*1.25=(80+8+0.8)*1.25=80*1.25+8*1.25*0.8*1.28=100+10+1=111
7/2.5=7*4/(2.5*4)=28/10=2.8
2.3/1.25=2.3*8/(1.25*8)=18.4/10=1.84
5.6/3.5=7*0.8/（7*0.5）=0.8/0.5=0.8*2/（0.5*2）=1.6