https://mathstodon.xyz/@mattp posted an interesting math puzzle on Mastodon yesterday. Since the original was in German, he provided a translation.
A train is pulling 11 passenger cars. There are 381 passengers total, and in any 3 consecutive cars there are 99 passengers. How many are in the ninth carriage?
https://epsilon-ix.masto.host/@mattp/109411569918653609
https://www.spektrum.de/raetsel/wie-viele-fahrgaeste-sind-im-zug/2071047 is the original article. A translation is available at https://www-spektrum-de.translate.goog/raetsel/wie-viele-fahrgaeste-sind-im-zug/2071047?_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp .
I solved the problem, but I didn’t use the technique described in the original article. I’ll post my solution later.