Bài giải:

Theo đề bài ta có:

N : 5 = a dư 0 -> N = a x 5

N : 9 = b dư 0 -> N = b x 9

a – b = 4

Suy ra: a x 5 = b x 9

              a x 5 – b x 9       = 0

a x 5 – ( b x 5 + b x 4 ) = 0

a x 5 – b x 5 – b x 4        = 0

5 x ( a – b ) – b x 4         = 0

          5 x 4 – b x 4           = 0

                        b x 4          = 20

                        b                = 5

Vậy N = 5 x 9 = 45