Just a note on the difficulty of getting 40% (for example) on the JLPT...
For the kanji/vocabulary section, kfmefe04's table is probably pretty close to the mark; you could probably get 40% knowing just 20% of the actual content because most of that section of the test just requires you to match readings with kanji.
But for the listening and reading sections, where you actually need to understand language, there's a certain minimum critical mass of knowledge you need to have, below which, whatever you know is close to worthless. Ever try reading a paragraph with 80% of the words blanked out? If you only understand 20% of the vocabulary & grammar in a reading or listening passage, it's quite likely you won't understand any of the meaning. I think knowing about 60% of the material is about the minimum you can scrape out a pass with.
For the kanji/vocabulary section, kfmefe04's table is probably pretty close to the mark; you could probably get 40% knowing just 20% of the actual content because most of that section of the test just requires you to match readings with kanji.
But for the listening and reading sections, where you actually need to understand language, there's a certain minimum critical mass of knowledge you need to have, below which, whatever you know is close to worthless. Ever try reading a paragraph with 80% of the words blanked out? If you only understand 20% of the vocabulary & grammar in a reading or listening passage, it's quite likely you won't understand any of the meaning. I think knowing about 60% of the material is about the minimum you can scrape out a pass with.

