Hello There!
The answer to your problem is 10:5 or 2:1
How to get 10:5
To get 10:5 you divide a number that can go into 20 and 10 which is 2. You divide 20/2 and 10/2 and get 10:5.
How to get 2:1
To get 2:1 you divide a number that can go into 10 and 5 which is 5. You divide 10/5 and 5/5 and get 2:1.
Hope This Helps!