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