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а * (18 : 9) = 20                                  9 * 4 + у = 100

а * 2 = 20                                            36 + у = 100

а = 20 : 2                                             у = 100 - 36

а = 10                                                  у = 64

Проверка: 10 * (18 : 9) = 20               Проверка: 9 * 4 + 64 = 100

                  10 * 2 = 20                                           36 + 64 = 100

                   20 = 20                                                100 = 100