How do I do megapixel crop math?

For example: If I have a 50mm prime, but I want to be able to crop in post to 75mm, a factor of 1.5 while retaining a resolution of say 20mp, all I'd need to do is multiply 20*(3/2) for a value of 30mp, since that is the ratio of 75mm/50mm, is that correct?

I'm thinking about changing up the way I shoot and switching over from 2 zoom lenses, to 2 prime lenses and I want to make sure I can cover all of my usual focal lengths by crop in post.

I am not entirely clear on how this effects compression. It seems like taking a 20mm and cropping to 40mm, you would still end up the compression aesthetic of the 20mm, but I've heard people say otherwise.