Howdy howdy. The 10日／十日 vocabulary term is kicking my butt. I know it has a long vowel — and, every time, I’m tempted to spell it とうか. But WaniKani (and other sources is ) have the correct reading/spelling as とおか.
I find this additionally strange because 8日 = ようか with 「おう」. Maybe there’s an etymology difference behind 八 and 十?
Of course we have exceptions, like おおき／大, to the long-お-vowel-takes-う “rule” (東京/とうきょう, 王／おう, 学校／がっこう). But is that right? Is that even a rule? Is 大き the exception or is there another dependency? And can I get a durtle badge for this?