For me, for most everyday math I use either google (Command+0 mapped to "new chrome tab") or the mac calculator (command+space for Spotlight, type "calc").
For larger personal math stuff, like my budget, I use Google Sheets.
I stay away from using REPL for math for the known issues that come with float handling decimals:
>>> 0.1 + 0.2
Our base-10 system isn't without problems (i.e. 1/3), but obviously we're all much more familiar with them.
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