Answer :
12 is the width because a desk is usually a rectangle so if the length on one side is 15 the other is also 15. So 15 + 15= 30. Then if you subtract 55 by 30 you get 24. Nd there's two side so if u do 24 divided by two u get 12
[tex]Perimeter=2width+2length\\\\
length=15\\\\
perimeter=54\\\\
54=2*15+2width\\\\
54=30+2width\ \ \ | subtract\ 30\\\\
24=2width\ \ | divide\ by\ 2\\\\
width=12inches
[/tex]
