Bài giải:

Theo đề bài ta có:

N : 6 = a dư 0 -> N = a x 6

N : 9 = b dư 3 -> N = b x 9 + 3

a – b = 2

Suy ra: a x 6 = b x 9 + 3

              a x 6 – b x 9       = 3

a x 6 – ( b x 6 + b x 3 ) = 3

a x 6 – b x 6 – b x 3        = 3

6 x ( a – b ) – b x 3         = 3

          6 x 2 – b x 3           = 3

                         b x 3          = 12 – 3

                         b x 3          =  9

                         b                 = 3

Vậy N = 3 x 9 + 3 = 30