3.21×8.2＋87×0.18＋0.82×54.9=3.21×8.2＋8.2×5.49＋87×0.18=8.2×(3.21+5.49)+8.7×1.8=8.2×8.7+8.7×1.8=8.7×(8.2+1.8)=8.7×10=87321.321÷321×3.21=321.321×3.21÷321=321.321×0.01=3.21321第二道的第一步：=321.321×3.21÷321其中3.21÷321可以加（）么？什么意思？