Math Is Fun Forum

aric

Member

Registered: 2013-05-03

Posts: 2

Z-score problem (percentile related)

plz help me. solution to the problem with fig. would be great. i'm trying to understand rather than just solve problems.

1. Assume that IQ scores are normally distributed with mean 100 and SD 15. Find the value which is the IQ separating top 30% and bottom 70%.

2. Let weight of men are normally distributed with mean 172 lbs and standard deviation 29 lbs. Find the value of the weight (X) which separate the top 0.5% with the bottom 99.5% of the weight.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,626

Re: Z-score problem (percentile related)

hi aric,

You haven't said if you understood the help you received in your last post.  I'm going to assume you did and carry on from there.

Both of these problems can be solved using a similar diagram  My picture below shows a standard normal distribution with two blue lines.  I haven't tried to put them in the right places for either problem but the method is what I think you need.

If you work out the probability of being in the area to the left of line 1, and also the probability of being to the left of line 2, the difference will tell you the probability of being between the lines.

In both questions that should take care of it.

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob