To find the length of \( d \), we use the three-dimensional distance formula. The points we will use are:
- The vertex at the back right side can be represented by the coordinates \( (l, w, 0) = (30, 20, 0) \).
- The vertex at the front left side can be represented by the coordinates \( (0, 0, h) = (0, 0, 18) \).
The distance \( d \) between these two points can be calculated using the formula for the distance between two points \((x_1, y_1, z_1)\) and \((x_2, y_2, z_2)\):
\[ d = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2 + (z_2 - z_1)^2} \]
Plugging in the coordinates:
\[ d = \sqrt{(0 - 30)^2 + (0 - 20)^2 + (18 - 0)^2} \]
Calculating each term:
- \( (0 - 30)^2 = 900 \)
- \( (0 - 20)^2 = 400 \)
- \( (18 - 0)^2 = 324 \)
Now summing these values:
\[ d = \sqrt{900 + 400 + 324} = \sqrt{1624} \]
Now calculating \( \sqrt{1624} \):
\[ \sqrt{1624} \approx 40.31 \]
Rounding this to the nearest tenth gives:
\[ d \approx 40.3 \text{ in.} \]
Thus, the length of \( d \) is approximately 40.3 in.