199×99 =199x(100-1)=19900-199=19700+200-199=19701
1025÷25×（28+75）=(1000+25)x1/25x(100+3)=41x(100+3)=4100+123=4223
0.25×78+2.2×2.5=2.5x7.8+2.2×2.5=2.5x(7.8+2.2)=25
0.3÷9/7+4.2×7/9=0.3×7/9+4.2×7/9=(0.3+4.2)×7/9=9/2x7/9=7/2
8/9×{15/16+（7/16-4）} = 8/9×(15/16-7/16+4)= 8/9×(15/16-7/16+4)=8/9×(4+1/2)=8/9×9/2=4
x/0.6-2/3=1(先去分母,等式左右同时乘以15) ,9x-10=15,x=25/9