this is a very light coding environment, but I turn to it often when working on light math, dividing up costs of trips, napkin math for finances etc. To me, what is so powerful is that, in plain text you can define the value of a variable as well as state its type: ‘$200.00 in CAD’ etc, and that the right gutter shows calculations, live. The app in the screenshots is Numi. I know https://soulver.app/ is a comparable app as well —

There's also Frink.