To find the length \( d \), we'll assume you are referring to a diagonal in a bounding box defined by the given dimensions \( l \), \( w \), and \( h \). The length of the diagonal \( d \) of a rectangular prism can be calculated using the formula:
\[ d = \sqrt{l^2 + w^2 + h^2} \]
Given:
- \( l = 30 \) inches
- \( w = 20 \) inches
- \( h = 18 \) inches
Substituting these values into the formula, we have:
\[ d = \sqrt{30^2 + 20^2 + 18^2} \]
Calculating each term:
\[ 30^2 = 900 \] \[ 20^2 = 400 \] \[ 18^2 = 324 \]
Now sum these values:
\[ 900 + 400 + 324 = 1624 \]
Now take the square root:
\[ d = \sqrt{1624} \]
Using a calculator, we find:
\[ d \approx 40.3 \]
Thus, the length of \( d \) rounded to the nearest tenth is:
\[ \boxed{40.3} \text{ inches} \]