Question

How can I add two 1-digit numbers with sums of 20

Answer 1

9+9 is the close to get 20 cause there isn't a way to make 20 with only single digits

Answer 2

It is not possible for sums up to 20, you can only add up to 18.

Explanation:

At the maximum, 9+9, you can get 18. It is not possible without going above one-digit.

Unless you mean repeating numbers (2+2+2 etc.), it is not possible to do so.