Your bedroom wall is 221 inches wide. You want to hang two posters, each 28 inches across, with an equal space between each poster and the wall edges.
How many inches of space should you leave between the wall edge and each poster?
First, calculate the combined width of the two posters: 28 inches/poster * 2 posters = <<28*2=56>>56 inches
Then calculate the remaining wall space after the posters are hung: 221 inches - 56 inches = <<221-56=165>>165 inches
Finally, calculate the space to leave on either side of the posters by dividing the remaining wall space by 3: 165 inches / 3 = <<165/3=55>>55 inches. Answer: \boxed{55}.