what is df?

how are z and t curves different? how are they similar?

if you are given the area if the tail of a t graph, how do you find the t alpha?

Measurments of acidity of rain water samples were recorded at 5 sites in a region, 3.5, 5.1, 5.0, 3.6, 4.8 (Ex = 22, Ex2= 99.26) give a 90% CI for the mean acidity of rain in that region