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

• Oct 17th 2007, 07:12 AM
beetz
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!
• Oct 17th 2007, 07:57 AM
beetz
is it P(random variable X < 120) for below 120? and P(X>120)
• Oct 17th 2007, 08:00 AM
topsquark
Quote:

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
• Oct 17th 2007, 08:15 AM
beetz
z-score for 120 is 1.25

i will be using a ti 83 to calculate this..
• Oct 17th 2007, 12:08 PM
beetz
the z-score isn't my final answer, right?
• Oct 17th 2007, 03:00 PM
beetz
help!!!
• Oct 17th 2007, 03:13 PM
topsquark
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
• Oct 17th 2007, 03:22 PM
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?
• Oct 17th 2007, 04:17 PM
topsquark
Quote:

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