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4.05-2.8-0.7 ＝4.05-（2.8+0.7）＝4.05-3.5=0.55
12×（1/4+1/6-1/3）=12*1/4+12*1/6-12*1/3=3+2-4=1
16÷2.5 =16÷10*4=1.6*4=6.4
0.7+3.9+4.3+6.1 =(0.7+4.3)+(3.9+6.1)=5+10=15
69×5/23 =69/23*5=3*5=15
（51+34/35）÷17 =51÷17 +34/35÷17 =3+2/35=107/35
38×1/4+17×0.25+45×25% =(38+17+45)*0.25=100*0.25=25
9999×7778+3333×6666 =99990000
32/33×29 =29*(1-1/33)=29-29/33=28又4/33