You will need a pen and paper for this and it will blow some minds. You need some basic math but its really simple!!!
You grab the paper and tell the HB to write down 5 numbers.
example:
8 2 3 1 4
Now you grab the paper and write down on the other side the outcome. ( I will get there on how you know the outcome )
Now under her 5 numbers you let her write anouther row. After her row you add one then she and then you. Till you have 5 rows.
example:
1) 8 2 3 1 4 (her numbers)
2) 9 4 3 7 7 (her numbers)
3) 0 5 6 2 2 (your numbers)
4) 2 4 5 3 2 (her numbers)
5) 7 5 4 6 7 (your numbers)
After this is done you tell her to add all the numbers.
-----------------
answer: 28 2 3 1 2
And surely this is the exact number you have written down on the back!
And this is how you do it.
When she writes down her first row of numbers you already know the outcome because row 2 till 5 will be 18 everytime. When she adds her second row just make your row so all the numbers are 9 combined. She puts down 1 you put down 8, she 2 you 7 etc
I hope this is all understandable
Have fun