に決まっている

Usage

(V/いA/なA~~-だ~~/N~~-だ~~) + に決まっている

Used to express a conjecture about someone definite outcome based on the speaker's conviction

Can be used if there is no solid basis for believing this is the case

紙に書いても忘れるに決まっている He is sure to forget even if he writes it down on paper

医者になりたいけど、無理に決まっている I want to be a doctor, but I'm definitely not cur out for it