In episode 20 of Code Geass: Hangyaku no Lelouch R2, there's a song playing at 4:20.
What is this song?
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I believe it is Strange Girl by Hitomi Kuroishi. I did a bit of research and reverse audio searching and eventually ended up in a wikipedia article which seemed to confirm this. I found the song so you can double check it in case I got the wrong music accidentally.