To find the length of \( d \) in the three-dimensional rectangular prism using the Pythagorean theorem, we'll apply the theorem in two stages.
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First, we find the diagonal length across the base, which we'll call \( x \). This can be calculated using the lengths of \( l \) and \( w \): \[ x = \sqrt{l^2 + w^2} \] Given \( l = 30 , \text{in} \) and \( w = 20 , \text{in} \): \[ x = \sqrt{30^2 + 20^2} = \sqrt{900 + 400} = \sqrt{1300} \approx 36.1 , \text{in} \] This aligns with the given value for \( x \).
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Next, we find the diagonal length \( d \) using the length \( x \) and the height \( h \): \[ d = \sqrt{x^2 + h^2} \] With \( h = 18 , \text{in} \): \[ d = \sqrt{(36.1)^2 + (18)^2} = \sqrt{1300 + 324} = \sqrt{1624} \]
Now, we calculate that: \[ \sqrt{1624} \approx 40.3 , \text{in} \]
Thus, rounding to the nearest tenth, the length of \( d \) is: \[ \boxed{40.3} , \text{in} \]