Stats question , have work and answer shown

[deaky220]

Homework Statement

i didnt know which forum stats is posted in so i posted it here, but the problem says the q is , Sheilas measure glucose level one hour after a sugary drink varies according to the normal distribution (u= 125 mg/dl) and r=10 mg/dl. What is the level L such that there is a probability only 0.05 that the mean glucose level of 4 test results falls above L?

Homework Equations

x= u(mew) + z*r

The Attempt at a Solution

I did a backwards normal calculation of x= 125 + .5199 (from table A probs) * 10 = 130.199

i think its right but i dunno, does the 4 play a part in my equation or is this the wrong way to do it? ANY information would be helpful

 

[HallsofIvy]

Yes, it does. The average of n samples from a population with mean mu and standard deviation \sigma is normally distributed with mean mu and standard deviation \sigma\sqrt{n}. Notice that means that the larger sample you have the smaller its standard deviation is and the more "accurate" it is.