To find the length of \( d \) in the given three-dimensional rectangular prism, we can use the 3D distance formula. This distance represents the diagonal line from the back right vertex to the front left vertex of the prism.
Given:
- Length \( l = 30 \) in
- Width \( w = 20 \) in
- Height \( h = 18 \) in
The coordinates for the two vertices are as follows:
- Back right vertex \( (l, w, 0) = (30, 20, 0) \)
- Front left vertex \( (0, 0, h) = (0, 0, 18) \)
Now, we can apply the 3D distance formula, defined as:
\[ d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2 + (z_2 - z_1)^2} \]
Substituting in our points:
- \( (x_1, y_1, z_1) = (30, 20, 0) \)
- \( (x_2, y_2, z_2) = (0, 0, 18) \)
Substituting into the formula gives:
\[ d = \sqrt{(0 - 30)^2 + (0 - 20)^2 + (18 - 0)^2} \]
Calculating each term:
- \( (0 - 30)^2 = 900 \)
- \( (0 - 20)^2 = 400 \)
- \( (18 - 0)^2 = 324 \)
Now summing those values:
\[ d = \sqrt{900 + 400 + 324} = \sqrt{1624} \]
Calculating the square root:
\[ \sqrt{1624} \approx 40.3 \]
Thus, the length of \( d \) rounded to the nearest tenth is:
\[ \boxed{40.3} \text{ in.} \]