general Maths Puzzle

There is a 4-character code, with 2 of them being letters and the other 2 being numbers.
How many maximum attempts would be necessary to find the correct code? Note that the code is case-sensitive.

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general Other Question

There are 3 ants at 3 corners of a triangle, they randomly start moving towards another corner.
What is the probability that they don't collide?
How many possible combinations are there in a 3x3x3 rubics cube?
In other words, if you wanted to solve the rubics cube by trying different combinations, how many might it take you (worst case senerio)?
How many for a 4x4x4 cube?