ps11.py

from random import choice

RUNS = 1_000_000

first_3 = []

def mean(lst):
    return sum(lst) / len(lst)

for i in range(RUNS):
    n_rolls = 0
    while True:
        roll = choice([1, 2, 3, 4, 5, 6])
        n_rolls += 1
        if roll == 3:
            first_3.append(n_rolls)
            break
        if roll in [4, 5, 6]:
            break


print("average: {}".format(mean(first_3)))