Question 102985
You're on the right track. Here's a hint. 

Number of 5 cent stamps = x
Number of 13 cent stamps = 2x-1
Number of 18 cent stamps = 2x-1+5

Value of 5 cent stamps = 0.05*x
Value of 13 cent stamps = ?
Value of 18 cent stamps = ?

Get a value equation for each stamp category.
Then plug those into an equation for the total value of the collection.
Total value of all of the stamps = $2.60
{{{highlight(0.05*x) + highlight(Value of 13 cent stamps) +highlight(Value of 18 cent stamps)=2.60}}}
Solve for x. Then you know the number of stamps in each category. 
Keep going and if you get stuck post another question.