One sum per page, from 1-20 in a single Excel Sheet. Select one and <Print> <Selection> will print one page; <Print> <worksheet> will print all 20.
The student may need to make a vertical chart of all possible pairs for reference.
E.g., for 7: 0+7, 1+6, 2+5, 3+4, 4+3 5+2 6+1, 7+0.
Each time you reopen the page the random numbers are re-generated, allowing for you to create different pages to avoid copying. However, that results in different numbers of possible pairs found. Recommended pace: 1 sheet per day.