Statistics homework problem??I need some help on this one, please!!

For each question you need to determine the z score. The formula is

z = (x - mean)/standard deviation

They've made it easy for you by making the SD=1. Once you have the z score, look up the cummulative distribution for that z score in a cum prob table (or use the function NORMDIST in Excel and just enter the value of x, the mean, the standard deviation and then TRUE for cummulative.)

Use this approach over and over. As an example, for the first one, find the cum prob for 7.5 and substract the cum prob for 2.5. In part b, multiply the probabilities by 200 to get the number of expected users.

so for part a) it would be 7.5-2.5=5
z=x-mean divided by standard deviation, that would be 5-5/1= 0?? seems like I did something wrong still??

You aren't using the standard normal distribution

x is variable
mean is mu (average)
sigma is standard deviation

Z (2.5)=((2.5-5)/1)=-2.5
Z(7.5)=((7.5-5)/1)=2.5

Now use the Standard normal distribution and look up the values corresponding to these two Z values.

The first will be the value (fraction of total population) of those who rated is less than 2.5 and the second is value (same fraction) those who rated it less than 7.5.

The difference is the value (fraction of population) in the desired range.

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