Explain how a bar model can be used to show a subtraction problem

The bar model is a pictorial representation of a problem.
In a word problem the bar model helps students to visualize the relationship between the numbers given and what are they looking for.
For subtraction ( and addition ) you can use Part - Whole bar model.
Whole - Part 1 = Part 2.
For example: There are 120 sheep on a farm and there are 20 cows fewer than sheep. How many cows are on a farm?
The bar model:
            Whole: 120


            Part 1                           20
120 - 20 = 100 cows.