Where did you get asked such question? It's an interesting one but it also feels more like a brain teaser that an investment bank would ask to watch you squirm vs. an actual market sizing where you can build a logic with assumptions.

Hello !

This is a GMAT question, not one you would get asked in an MBB interview, 99% sure.

In any case, you will need to first rescue your combinatory math knowledge from high school/uni to calculate the number of combinations that are possible.

Then, you would need to reduce that number, since most of those wouldn´t make sense in English, by applying a % reduction -that you should justify by reasoning with number of vowels/non vowels that a normal word has, etc.-.

Again, this one is too hard to worry about it.

Hope it helps!

Cheers,

Clara