s(x) = 1.7 s(y) = 1.4

x-bar = 6 y-bar = 7.2

n(x) = 40 n(y) = 50

Find a 99% CI for the difference between mean x and mean y.

Now I solved this with a t distribution (which according to the key in the book is correct), but I don't understand why the central limit theorem can't be used so that I can use z-value from the cumulative normal table instead? Could anyone help explain why?