Let's use the variable x to represent the spaces between posters.
So the spaces at the left and right of the group will be 3x.
Drawing the diagram, we have:
Writing an equation to solve for x, we have:
[tex]\begin{gathered} 3x+3x+1.5+1.5+1.5+1.5+1.5+x+x+x+x=15 \\ 6x+7.5+4x=15 \\ 10x+7.5=15 \\ 10x=15-7.5 \\ 10x=7.5 \\ x=\frac{7.5}{10} \\ x=0.75 \end{gathered}[/tex]So the space between each pair of posters is 0.75 feet.