I don't know about other firms, I only prepared for MBBs.
My background is Mechanical Engineering, therefore you can bet I didn't have any clue whatsoever on what NPV was.
I wouldn't even bother learning it. MBB interviews are about how you reason, how your mind works (+ a FEW frameworks and key numbers), not about the formulas you know. Thus basic math ok, you could also be asked the perimeter or a circle or the surface of a box, but no super-complicated formulas.
To get better at mental calculations, there is no easy way. You have to practice, try to invent math calculations and solve them in your mind. That's the only way, unfortunately :)
However, if you struggle with quant problems in general, maybe there is a chance you haven't understand them completely. In that case, I would suggest talking to someone (coach, friends, peer, ...) and try to understand them together.