To find the width of the rectangular prism, we can use the volume formula:
\[ V = lwh \]
Where:
- \( V \) is the volume
- \( l \) is the length
- \( w \) is the width
- \( h \) is the height
We know the following values:
- \( V = 90 , \text{ft}^3 \)
- \( l = 3 , \text{ft} \)
- \( h = 12 , \text{ft} \)
Substituting the known values into the formula:
\[ 90 = 3 \times w \times 12 \]
Now, we can simplify the right side:
\[ 90 = 36w \]
Next, we will solve for \( w \) by dividing both sides of the equation by 36:
\[ w = \frac{90}{36} \]
Calculating \( \frac{90}{36} \):
\[ w = 2.5 \]
Thus, the width of the rectangular prism is:
\[ \boxed{2.5} , \text{ft} \]