脱式计算，能简便的写简便过程．
6.3+6.44+2.7+3.56                600÷12                      （58+420÷6）×12
（450﹣125）×（82﹣64）         72.8﹣8.66+5.34              （200+350）÷11×38
720﹣（750÷15+248）             11.8+391÷17+25．