To find the width of the rectangular prism, we can rearrange the volume formula \( V = lwh \) to solve for width \( w \).
Given:
- \( V = 90 , \text{ft}^3 \)
- \( l = 3 , \text{ft} \)
- \( h = 12 , \text{ft} \)
We can plug in the values into the formula and solve for \( w \):
\[ 90 = 3 \times w \times 12 \]
Calculating the product of length and height:
\[ 90 = 36w \]
Now, we can solve for \( w \) by dividing both sides by 36:
\[ w = \frac{90}{36} \]
Calculating the fraction:
\[ w = 2.5 \]
Thus, the width of the rectangular prism is \( \boxed{2.5} \) ft.