10

from itertools import product
a = list(map(''.join, product('ABCX', repeat=5)))

#   Первый способ
words = []
for i in a:
    if i.count('X') == 1 and i[-1] == 'X' or i.count('X') == 0:
        words.append(i)
print(len(words))

# Второй способ
print(len(list(filter(lambda x: x.count('X') == 1 and x[-1] == 'X' or x.count('X') == 0, a))))