Number of ways I can fit rectangles within a longer rectangle

Say I have a long rectangular hole of size n and then a bunch of smaller rectangles of different lengths a, b, c, etc such that the sum of their lengths is <=n.

I am trying to figure out the number of distinct permutations. For instance, if I had a hole of length 4 and two rectangles of length 2, there's only one way to arrange that. But if I had one of length 3 and one of length 1, that's two ways to arrange it.

If I had a hole of length 4 and a sub-rectangle of length 2, there are 3 ways, and so on.

Edit for clarification: The lengths are integers only, and they can only be "snapped" in at integer intervals (so you can't place a rectangle at, say, 1.233249 and then claim there are infinitely many ways to insert a rectangle). Area also doesn't matter... the height of the rectangles are all the same. It's only the lengths that differ.

Is there a simple mathematical combinatoric that can be applied here?

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For clarity: so $n$ is a length, not the area of the rectangle and you want a solution to the discrete problem (ie everything has integer length and can only be placed at integer positions not at 2.54234...)? – example Apr 3 '12 at 13:54
Yes, exactly right. Edited the original post for clarity. – WhatsInAName Apr 3 '12 at 13:57
What do you mean by a hole of length 4 and sub- rectangle of length 2, there are 3 ways? – Long Mai Apr 3 '12 at 14:08
Say the hole looks like this: oooo. The rectangle of length 2 looks like this: RR. There are three ways to arrange: RRoo, oRRo, ooRR – WhatsInAName Apr 3 '12 at 14:09