The scale factor in a 1:20 drawing is represented as $1/20$ or $0.05$.

In a drawing with a scale of $1:20$, each unit of measurement depicted corresponds to $20$ units of the same measurement in reality. For instance, if a dimension of $1$ centimeter appears in the drawing, it reflects an actual length of $20$ centimeters in the real world. This scaling method is particularly useful for creating manageable representations of large objects or spaces, such as buildings or landscapes, on paper.

To determine the actual size from the drawing, you can multiply the measurement shown in the drawing by the denominator of the scale factor. For example, if a wall measures $3$ centimeters in the drawing, its real length would be calculated as:

Conversely, to create a scaled drawing from real-life measurements, you divide the actual measurement by the denominator of the scale factor. Therefore, a wall that is $100$ centimeters long in real life would be represented in the drawing as:

Understanding scale factors is essential in various fields such as architecture, engineering, and map reading. They facilitate accurate and proportional representations, making it easier to visualize and plan projects. It’s important to remember that the scale factor is always a ratio. In this case, a scale of $1:20$ indicates that the drawing is $20$ times smaller than the actual object.