License Plate Combinations: Clarifying the Math

[requied]

Yes.

I guess there is no a similar application at letters order.

 

[haruspex]

I guess there is no a similar application at letters order.

Quite so, that was easier. Had you only been told there was at least one A then, depending on your approach, there would have been a risk of double counting.
(But in that specific case the easy way is 26³-25³.)