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:
![18\text{ ft} = 216\text{ in}](https://img.qammunity.org/2020/formulas/mathematics/middle-school/42rrfdl55tqbpf6e9p9hntqlc5rco0cjby.png)
So, the scale is
![(216)/(6.4) = 33.75](https://img.qammunity.org/2020/formulas/mathematics/middle-school/wnqpzp1w6kxuwg4pnp6ytjusuociufmpxk.png)
which means that one inch in the plan is the same as 33.75 inches in the real world.