Countless moons ago, the arrival of the iPad provoked a light-hearted rant about the fact that the percent key on computer-based calculators was broken (at least according to my understanding of how basic maths is supposed to work).
Ambiguous sums just don't add up (me, Aug 2010)
Well Microsoft have only gone and open-sourced calc.exe, so now there's no excuse - I can finally stop whinging and fix the problem myself :-)
Announcing the Open Sourcing of Windows Calculator (Microsoft, Mar 2019)
For a more serious explanation of why the percent key works the way it does, here's a link to Raymond Chen's excellent blog:
How does the calculator percent key work? (Raymond Chen, Jan 2008)