As I understand it, "you are 250" means that you are a simpleton. From a little research, it comes from the fact that Chinese coins used to have a hole in the middle so that they could be strung together in amounts of 1000 (called a diao (吊). The term ban diao zi (半吊子), or half a diao, was used to mean someone not having full knowledge. It was used to describe oneself in a humble manner and not necessarily negative. However, half of a half diao - 250, or er bai wu (二百五) was half of a half wit - definitely an insult. The equivalent English slang term would be "not playing with a full deck (of cards)".

idiot haha

i like fdmaxey's answer and chinese has a lot of these idioms that are hard to remember if you don't use them in a everyday basis. but here, maxey and cheer are saying 205, unless it is a colloquial form as it is understood that after hundreds comes tens, but i am not sure it works automatically that way. i used to be familiar with chinese 25 years ago, but now i almost forgot everything.

Hello Cappuci , "er bai wu" ,it is sort of an insult : you are 250 = you are foolish.

no idea