In a srs of 80 teenagers, the average number of text messages received per day is found to 50. if the standard deviation of number of texts received is known to be σ = 12, find the 95% confidence interval for the number of text messages received.

To solve this problem you have to find out what the value of +/- Z is that encloses 95% of the area under the normal distribution curve. For this you can use this formula: Please see the attached image.

Find the upper and lower limits of x, corresponding to +Z and -Z respective.

I hope this helps, Regards.