To find the width of the rectangular prism, we can use the volume formula:
\[ V = l \times w \times h \]
We know:
- \( V = 90 , \text{ft}^3 \)
- \( l = 3 , \text{ft} \)
- \( h = 12 , \text{ft} \)
We need to solve for \( w \):
\[ 90 = 3 \times w \times 12 \]
First, calculate \( 3 \times 12 \):
\[ 3 \times 12 = 36 \]
Now we can rewrite the equation:
\[ 90 = 36 \times w \]
To isolate \( w \), divide both sides by 36:
\[ w = \frac{90}{36} \]
Now simplify the fraction:
\[ w = \frac{90 \div 18}{36 \div 18} = \frac{5}{2} = 2.5 \]
Therefore, the width \( w \) is:
\[ \boxed{2.5} , \text{ft} \]