To count on, I always take the larger number in my sum and add the second number.
I can add on by counting up in my head and/or using my fingers.
For example, if the calculation is 13+4. I start with the number 13 in my head, put up four fingers and count
1 - 14
3 - 16
4 - 17
Then I know the answer is 17.
Have a go at these calculations below: