Back of the Envelope Math for AI Systems

1. The Estimate Is the Interview

Classic system design interviews already have a named step for this: back-of-the-envelope estimation, rough math done out loud to size a system. AI system design needs that same skill even more, not less, because the costs involved are bigger and far less intuitive. The candidate who can say "that's about 10 billion tokens a day, roughly $30K, so caching is not optional" owns the room. The candidate who just waves a hand and says "it'll be expensive" does not.

Think of this lesson as a drill. Five numbers worth memorizing