The common multiple of 12 and 20 is 60.
You've got to multiply the first one by 5 and the second by 3.
5*5= 25
This therefore becomes 25/60
The second becomes 3*3= 9
This becomes 9/60
As you're subtracting, you only subtract the numerators.
25-9= 16
Therefore, the answer is 16/60 which can be simplified to 4/10 or 2/5
Hope this helps :)
