To find the capacity of the drainage trench shaped like an upside-down rectangular pyramid, we can use the formula for the volume of a pyramid:
\[ \text{Volume} = \frac{1}{3} \times \text{Base Area} \times \text{Height} \]
In this case:
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The base area is the area of the top opening, which is given as 30 ft wide by 250 ft long: \[ \text{Base Area} = 30 , \text{ft} \times 250 , \text{ft} = 7500 , \text{ft}^2 \]
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The height is the depth of the trench, which is given as 15 ft.
Now, we can substitute these values into the volume formula: \[ \text{Volume} = \frac{1}{3} \times 7500 , \text{ft}^2 \times 15 , \text{ft} \]
Calculating this gives: \[ \text{Volume} = \frac{1}{3} \times 7500 \times 15 = \frac{112500}{3} = 37500 , \text{ft}^3 \]
Therefore, the capacity for the trench is:
\[ \boxed{37,500 , \text{ft}^3} \]