Freaking Out- NLN-PN entrance exam

I have been practicing for my entrance exam. I was doing the math portion of the test and go 30 out of 40 correct in the time allotted. That's a 75% percent, which to me is bad. So how much can I miss on the actual exam? Also, I heard some questions are not countable towards your score. My school accepts scores in the 90th percentile.