567219 = 30
- 30
--------
567189
We will suppose he crosses out 7, which makes the addition of the row,
minus that figure, 29. He gives you that result and you at once name the
crossed off figure. There are two ways of arriving at the answer. The
simplest and quickest way is to add the units in the result together
until only one figure remains and deduct it from 9. For instance, we
will take 29. Add the 2 and 9 together, which make 11; add 1 and 1
together and you have 2, which deduct from 9, leaving 7, the figure
erased in the above example.
Supposing 1 was the figure erased, the addition of the remaining figures
would then be 35; 3 + 5 = 8, 9 - 8 = 1, the figure crossed off.
The second method is to reckon the next multiple of 9 above the figures
given you; for instance, supposing they are 29, the next multiple of 9
is 36. Deduct 29 from it and it leaves 7, the erased figure. If either 9
or 0 is erased the result is the same. You can get out of the
difficulty, on being told you are wrong, by saying (in case you have
given 9), "Yes, I see it is a nought; I thought it had a tail, so
mistook it for a nine." If you have named 0 and it turns out to be 9,
you can say, "Oh, I didn't notice the tail; of course I should have said
nine."