Answer:
840
Step-by-step explanation:
He has 7 posters to chose from for the 1st spot
He chooses one, so he has 6 left
He has 6 to chose from for the second spot.
He chooses one, so he has 5 left
He has 5 to chose from for the third spot.
He chooses one, so he has 4 left
He has 4 to chose from for the last spot.
7*6*5*4 =840
