A Geometric variable counts how many trials it takes to get the first success, when each trial succeeds independently with probability p. k = 1 means success on the very first try.
Task: write geometric_pmf_mean(p, k) returning [pmf, mean], each rounded to 4 decimal places, where mean = 1/p.
k counts trials starting from 1, never 0 — there's no such thing as succeeding on your "zeroth" try.p) pushes the mean number of trials needed higher.