Your bedroom wall is 129 inches wide. You want to hang two posters, each 24 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?
To find out how much space you should leave between the wall edge and each poster, you need to subtract the total width of the posters from the width of the wall and divide it by 3 (to account for the space on both sides of each poster):
Total space to leave = (Width of wall - 2 * Width of poster) / 3
Total space to leave = (129 - 2 * 24) / 3
Total space to leave = (129 - 48) / 3
Total space to leave = 81 / 3
Total space to leave = 27 inches
Therefore, you should leave 27 inches of space between the wall edge and each poster.