Question 73913
Let the number of 5 cent stamps be x
Then the number of 13 cent stamps = 2x - 1
The number of 18 cent stamps = 2x - 1 + 5
                             = 2x + 4

Value of 5 cent stamps = 5x cents
value of 13 cent stamps = 13(2x-1)
                        = 26x - 13 cents
value of 18 cent stamps = 18(2x + 4)
                        = 36x + 72 cents

Total value is 5x + 26x - 13 + 36x + 72 = 260
==>        67x + 59 = 260
==>         67x = 260 - 59
==>         67x = 201
==>          x = 201/67
               = 3
so x = 3
Thus the number of 13 cent stamps = 2x - 1
                                  = 6 - 1
                                  = 5

Good Luck!!!