"2+2=80" is a parody of "Prime Minister Tōjō's Arithmetic", a Japanese war propaganda poster. See also this historical inquiry by Japanese blogger Ayukawa Reiji:
* http://blog.goo.ne.jp/ayureiji/e/dc34f96b4db8313488a4c69ddc55d4eaAugust 31stUshio.... What happened last week Monday again? This year we had Choukai-san help us again.Wasn't Oboro-chan in the docks?Every year we have the customary homework catch-up.There! You made a mistake in your calculation!