递等式计算（能简便的用简便方法）． 
12.5×（0.7×0.8）          70.8﹣0.8×0.5            13﹣3.79+6.21   
12.5×（40+0.7）           4.12﹣1.78﹣1.22          4.2÷0.7÷6．