eliminate a certain combination of letter and number

Welcome to the forum.

So what do you want to be returned in C if A contains A and B contains 1?

Hi, welcome to the forum.
Maybe this is a solution:

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where a1 can be between 65 and 73
Cheers
Erwin

So you've got an 8*8=64 outcome space. And some combinations thereof are invalid.

So the way I'd do it is create all those 64 combos down a column -- A1, A2, A3, ... I8-- and then go back and delete out whatever combos are invalid.
And then however long that range of cells is (50 cells? 55? whatever) set up a RAND that generates that many results as an integer, and run that as an index position to return from the range of valid results.

The other way to do it is to generate your 8*8 results, then compare that against a lookup table for "invalid" results, and either return a null for invalid or do a recalc, but that's starting to get into recursion and anyway I'm not sure it's the best thing to do.

So I think your best result would be to create a linear array of allowable results and then just feeding a random number into a position lookup in the array.

You could make it a little more sophisticated by having all 64 results and then a helper cell with a manual Boolean switch for whether it's included, and then use that to create a helper column that only includes allowable entries as the index for your random lookup. And you could automate the array creation by telling it how many letters and numbers to include and then having it march down with ROW to fill it out or whatever.