The scale is simply the ratio between the distances in the plan and the actual distances. So, first of all, let's convert all lengths to the same unit measure:
[tex]18\text{ ft} = 216\text{ in}[/tex]
So, the scale is
[tex]\dfrac{216}{6.4} = 33.75[/tex]
which means that one inch in the plan is the same as 33.75 inches in the real world.
