To find the length of the rectangle in Spencer's new scale drawing, we first need to determine the scale factor used in his drawing compared to the original rectangle.
The original dimensions of the rectangle are:
- Length = 20 inches
- Width = 12 inches
Spencer wants the new width to be 1.5 inches. We can find the scale factor by comparing the new width to the original width:
\[ \text{Scale Factor} = \frac{\text{New Width}}{\text{Original Width}} = \frac{1.5 \text{ inches}}{12 \text{ inches}} = \frac{1.5}{12} = \frac{1}{8} \]
This means that Spencer's new drawing is at a scale of \(\frac{1}{8}\) of the original size.
Now, we apply this scale factor to the original length to find the new length:
\[ \text{New Length} = \text{Original Length} \times \text{Scale Factor} = 20 \text{ inches} \times \frac{1}{8} = \frac{20}{8} = 2.5 \text{ inches} \]
Therefore, the length of the rectangle in Spencer's new scale drawing is:
\[ \boxed{2.5} \text{ inches} \]