Ok, consider the scale 1 : 100. Let's say the measurements on the exam paper for a particular question are given in metres. Keep in mind, a metre is 1,000 milimetres.
Now 1 : 100 means "for every 1 milimetre that you draw on the page, that represenets 100 milimetres in real life".
So you are scaling downwards the measurements given on the question. Therefore you are effectively dividing by 100. So 1 metre (1,000 mm) divided by 100 would become 10 mm on your page.
Similarly, if the scale was 1 : 50, you would be dividing the measuremnt on the question (in mm) by 50 to get what you must draw.
Hope this is of some help.

G4V1N—