Difficulty with a STAT170 GNAT question (1 Viewer)

[decoil]

Hey guys, I feel like a total idiot having to ask such a simple question, But for the life of me i cannot figure it.
It reads:

What is the 24th percentile of a normal population with mean = 166 and standard deviation = 68. (to the nearest integer).

Now this is taken from the macquare slides, and is infact a practice question for GNAT3. The book says the answer is 118.

What I keep trying to do is find the z score. The closest i can find on the table is 0.2389, being 0.71 zscore.
Now after this, from other chapters, it leads me to do mean + SD x zscore.

That being, 166 + 68 x 0.71, but this is far off the answer, being 214.28... so obviously im doing something wrong at some point, or perhaps using the wrong concept altogether >.>

Well if anyone can help me out... thanks alot

 

[Jennt]

Because the 24th percentile will fall below the 50th percentile you will need to assign the z-score as a negative. So your equation would be:

y = 166 + (68 x -0.71) = 117.72

I've been re-listening to my STAT170 lectures on the train to/from work this week and am pretty sure you will find this info in Lecture 4.

regards and hope this helps - the reason why I'm re-listening to the lectures is that I'm feeling a little challenged by Assignment 2...

 

[decoil]

Oh wow haha... now that iv heard that, it seems so obvious. Thanks alot for the reply, it got to the stage that i was using excel on the gnat all because i couldnt work it out