递等式计算，能简算的要简算．
（4.2﹣0.4×6）÷4                      （15300÷600+67.5）×180
10500÷（10500÷25×1.25）              （5.84﹣3.9）÷0.4+0.15×0.92
（2400﹣480×2.5）÷1.6                   0.32×12.5×250
2.4×5.5+2.4×3.5+2.4                   9.825×0.2+98.25×0.09﹣0.393÷0.4．