In how many ways can 4-digit numbers be formed using 2,2,8,8 if each digit is used once only?
I'm confused as to how to solve this problem. If the question was "How many ways can 4-digit numbers be formed using 2 and 8 if each digit can be used any number of times?", the answer should be $2 \times 2 \times 2 \times 2 = 16$, since there a 2 possible digits (2 and 8) that can be used for each of the 4 positions.
However I'm not quite sure of how to solve the problem in the title.