title: math.h - z-score from percentile
body: <p>I'm building an app in Objective-C for iOS that requires some statistics calculations. I read on <a href="https://stackoverflow.com/questions/2785944/cumulative-normal-distribution-function-in-objective-c">this thread</a> how to calculate percentile from a value, but how about going in the opposite direction? </p>
  
  <p>I have a percentile, and need to get the z-score from that (their example uses 5th percentile corresponds to -1.645). Which math.h function w/ formula can I use?</p>