# Thread: Help! Given normal distrib. w/ mean=100, stand. dev.=16. How to find p(below 120)?

1. ## Help! Given normal distrib. w/ mean=100, stand. dev.=16. How to find p(below 120)?

i'm not sure what to do! how would i find the probability below and also above 120?!

i greatly appreciate any help!

2. is it P(random variable X < 120) for below 120? and P(X>120)

3. Originally Posted by beetz

i'm not sure what to do! how would i find the probability below and also above 120?!

i greatly appreciate any help!
What's the z score for 120 for this distribution?

-Dan

4. z-score for 120 is 1.25

i will be using a ti 83 to calculate this..

5. the z-score isn't my final answer, right?

6. help!!!

7. The z score tells you how much of the distribution you need to take to figure out the probability. You can't calculate it on a TI-83: the calculator does not have that capability. You need to look it up in a table.

-Dan

8. holy cow. the text book never goes over a table....

there are problems like this in my textbook P(X<40)=normcdf(-E99,40,75,3)

would i just apply that here since i have no knowledge of using a table?

9. Originally Posted by beetz
holy cow. the text book never goes over a table....

there are problems like this in my textbook P(X<40)=normcdf(-E99,40,75,3)

would i just apply that here since i have no knowledge of using a table?
See one of your other posts. Apparently I was wrong about the TI-83. However, I am astonished that your instructor never taught you how to use the table and only expects you to be able to use your calculator...

-Dan