Question 21088
Let "x" be the number of 13cent stamps.
Then x = 2(# of 5cent stamps)-1.
So, the number of 5cent stamps =(x+1)/2
Number of 18cent stamps is x+5

The value of "x" 13cent stamps is 13x.
The value of the 5cent stamps is 5[(x+1)/2]
The value of the 18cent stamps is 18(x+5)

Equation:
  Value of 13cent + Value or 13cent + Value of 18cent = 260cents
     13x + 5[(x+1)/2] + 18(x+5) = 260
Solve for "x" to get your answer.

Cheers,
Stan H.