计算：

（1）7+（-2）-3.4；                                 （2）(-21.6）+3-7.4+(-2/5)                                  （3）31+(-5/4)+0.25                                                            （4）7-(-1/2)+1.5                                          

（5）49-(-20.6)-3/5                                                         （6）(-6/5)-7-(-3.2)+(-1)