Final answer:
To draw a rainfall graph using a 1cm to 20mm scale, plot rainfall in millimeters against time in centimeters, using a ruler to measure 1cm for every 20mm of rainfall.
Step-by-step explanation:
To draw a rainfall graph using a scale of 1cm to 20mm, you would plot the rainfall in millimeters against time in centimeters. This means that the x-axis of the graph represents time in centimeters, and the y-axis represents rainfall in millimeters. You would use a ruler to measure 1cm for every 20mm of rainfall. For example, if you have 40mm of rainfall, you would plot a point at 2cm on the x-axis.