To find the length of the rectangle in Spencer's new scale drawing, we can use the relationship between the original rectangle's dimensions and the desired width of the new drawing.
The original dimensions are:
- Length: 20 inches
- Width: 12 inches
In the smaller scale drawing, the new width is 1.5 inches. We can set up a proportion based on the original and new widths to find the new length.
Let's denote the new length as \( L \).
The relationship between the original and new dimensions can be expressed as follows:
\[ \frac{\text{new width}}{\text{original width}} = \frac{\text{new length}}{\text{original length}} \]
Substituting in the values we have:
\[ \frac{1.5}{12} = \frac{L}{20} \]
Cross-multiplying gives:
\[ 1.5 \times 20 = 12 \times L \]
Calculating the left side:
\[ 30 = 12L \]
Now, solve for \( L \):
\[ L = \frac{30}{12} = 2.5 \]
Thus, the length of the rectangle in Spencer's new scale drawing is:
\[ \boxed{2.5} \text{ inches} \]