You can just subtract them seperatly like this:
20-12=8
2/5-3/5= -1/5
Since its negative, you have to take 1 off of the final answer and put 4/5 so the answer is 7 4/5
Another way is making it a decimal
12 3/5= 12.6
20 2/5= 20.4
20.4-12.6= 7.8
7.8=7 4/5