Câu 1 : Lấy mẫu chung là 40,ta có:

15 / 40 + 10 / 40 - 16 / 40 = 15 + 10 - 16 / 40

                                      = 9 / 40

câu 2:  = 1 / 5 + 1 / 30 x 1 / 2

           = 1 / 5 + 1 / 60 = 12 / 60 + 1 / 60 = 13 / 60

                          25 . ( x - 57 ) = 25

                        x - 57 = 25 : 25

                       x - 57 = 1

                        x = 1 + 57

                   => x = 1