Question 49308
Let us put the problem statement into Equation:

Let:

Number of 5 Cents Stamps    = A
Number of 13 Cents Stamps   = B 
Number of 18 Cents stamps   = C

Now:
Number of 13 Cents Stamps   = B = (2A-1) The number of 13 cent stamps is one less than twice the number of 5 cent.

Number of 18 Cents stamps   = C = B+5 = 2A-1+5 = 2A+4 :The number of 18 cent stamps is five more than the number of 13 cent stamps. Note that I have converted interms of " A".

We know that : 5A + 13B + 18C = $2.6 = 260 Cents

In the above equation, substitute the values interms of "A" to get following equation.

5A + 13(2A-1) + 18( 2A+4) = 260

5A+26A-13+36A+72 = 260

67A-13+72 = 260

67A = 260+13-72 = 201

A = 201 / 67 = 3

Therefore B = 2A-1 = 2*3-1 = 5
Hence Number of 13 Cents stamps = B = 5
Also C= 2A + 4 = 2*3+4 = 10

Answers : A=3, B=5, C=10 We can verify the answers by substituting values in the following equation:

5A + 13B + 18C = 260 Cents
5*3 + 13*5 + 18*10 = 15+65+180 = 260

Thats all.....
B.Shridhar