Answer:
1/20 or 0.05
Explanation:
Assuming that all five posters are different, the number of ways to randomly place those posters is given by:

If we want the largest to be in the center and the next largest to be on its left, two poster will have a set position and only three could be randomized:

Therefore, the probability is:

The probability is 1/20 or 0.05.