[AccessD] Something too hard for me...

Hi all,
I am trying to create a little project to calculate some prices of a Radio
Station advertising list
In a table I keep CATEGORY and PRICE e.g.
10 --> 55,00EURO
20 --> 80,00
30 --> 90,00
bla, bla

What I want to calculate is the price of an uncategorized spots between two
categories.
25 SPOTS....
The calculation is:
A) the 20 spots cost 80,00
B) I have to select the next category (in that case 30) to find out how much
cost one spot (90,00/30 = 3,00EURO)
C). the 5 spots cost 5X3,00=15,00
so, the total price for the uncategorized query must be
80,00+15,00=95,00euro


Could anybody please help me?

Thank's a lot
Kostas Konstantinidis