The best puzzle game I've played in the jam so far! Both in mechanic and polish (release this and no one would think this is a jam game).
I think the best thing about this mechanic is that there's no interaction between the numbers (only between number & operator), so the inputs and outputs have a one-to-one coorspondance, cutting down on the complexity, thus enabling logical thinking (+aha moments) and avoiding excessive trial-and-error.