let

l = the length of the hallway

w = the width of the hallway

a hallway is 72 feet long

l = 72 ft

the length is less than 12 times the width

l < 12w

if we replace the value for l in the above inequality we get

72 < 12w divde both sides by 12

72/12 < 12w/12

6 < w

w > 6

the hallway is more then 6 feet width.