The scale factor between shapes is the number that when multiplied by the dimensions of the original shape will give the dimensions of the transformed shape.
Likewise when the dimensions of the transformed shape is divided by this number, it will yield the dimensions of the original shape.
The original shape has a length of 36km.
= 36 * 0.5
= 18 km
This proves that 0.5 is the scale factor because when the length of the original shape was multiplied by it, the result was the length of the transformed shape.