Not to be dramatic but they do send home 50% of those that come in for the interview. I'm assuming they don't come prepared... for the majority, if you don't honestly study a good amount, you will not pass. It's more than common aviation knowledge, it's a lot of definition based and formula based stuff that you need to be pretty brushed up on prior.

Edited:

I'm not actually sure how it is scored. I don't know that each test is scored individually... I have heard many times and perhaps even in the interview it was relayed to me that there is some cumulative score. A computer prints out a piece of paper when you hit submit at the end, and it says right then and there whether you P/F. No good answer there.