2*х=840 , х:19=19, х:3-18=12, 120:(25-х)=10, х*5+80=690, (708-х)+130=500                        поробная запись

2*х=840                                                      х*5+80=690

х=840:2                                                       х*5+80-690=0

х=420                                                          х*5-610=0

                                                                       х=610/5

                                                                       х=122

                                                                    120/(25-х)=10

                                                                     25-х=120/10

                                                                       25-х=12

                                                                      х=25-12

                                                                      х=13

х:19=19                                                          (708-х)+130=500

х=19*19                                                          838-х-500=0           

х=361                                                              338-х=0

                                                                        х=-(-338)=338

х:3-18=12

х:3-18+12=0

х:3-30=0

х=3*30

х=90