Question: I set the problem up, but when I solved for N I got 2.666 which is much too low for a sample size. The answer should be in the 30's or 40's I'm guessing. I set the problem up using the equation z= (x bar - mu) / (s/ square root N)
..where x bar is the sample mean, mu is the population mean, s is the standard deviation and N is the sample size.

The problem states: "Suppose the population of trees in Willow River Park is an average height of 100 inches with a standard deviation of 30 inches. The park ranger believes there is a section of the park in which tall trees cluster together and decided to sample the heights of trees in that area. Assuming the park ranger finds the average height of trees to be 136 inches, answer the following questions (assume alpha=.05)."

Knowing this, I set up 7a as 1.96= (136-100)/(30 over square root N). When I solved for N, though, I got 2.668, which is much too small of a value for N. Did I set this up wrong?  Edit

Answer: At alpha=0.05 , and z= 36/{30/sqrt(N)} =1.96

you set the problem right.

N=2.668=3  Edit