（1）1.56x+3.16=7.84

                 1.56x=4.68

                       x=3（2）1.6x+2x×0.7=18

               16x+1.4x=18

                           x=6

（3）（x-3）÷4=7.2

                     x-3=28.8

                        x=31.8

  （4）0.7×3-0.3x=0.06

              2.1-0.3x=0.06

                    0.3x=2.04

                         x=6.8