Example: a12345bcd
Note: there are 26 letters and 10 digits available
1) If numbers and letters can be repeated
We have 4 positions for letters and 5 positions for numbers.
So, we can choose 1 out of 26 letters to first position, 1 out of 10 numbers for each of next 5 positions, and 1 out of 26 letters for each of last 3 positions.
In this way we have: variants.
2) If no repeating is allowed.
In this case amount of numbers and letters we can choose will decrease by 1 on each step. (If we already have 'a' in the plate, we can't put it again)
Other solution. We can say, that we choose 4 different letters and 5 different numbers. And than we arrange them on their places.
Actually, I would not recoment this solution. It looks like round trip. But may be more convenient in some other tasks.
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