Valeria drew a scale drawing of a farm. The goat pen, which is 20 meters long in real life, is 5 millimeters long in the drawing. What is the scale of the drawing

sketch : real

= 5 mm : 20 m
= 5 mm : 20000 mm , (should have the same units)
= 5 : 20000 , (now I can drop the units by dividing each term by mm )
= 1 : 4000 , (I reduced the ratio to lowest terms)