11 × 3 ---- 33 + 1 note 1 ---- ÷ 2 34 17 × 3 ---- 51 + 1 note 2 ÷ 2 52 ---- 26 two 9's = 8 = 11
HOW TO NAME A NUMBER WHICH HAS BEEN ERASED
Request a member of the company to write a row of figures, the number of which is immaterial, add them together and subtract the addition from the row. Then to cross out any figure from the result, add the remaining figures together and give you the total, when you will tell him which figure he has erased. Of course, you do not see his figures and can leave the room while he makes them.
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."
A LESSON IN THE CORRECT FORMATION OF A FIGURE
Request a friend to write the following figures:--
Take the paper from him and, after pretending to scrutinise the row, ask him to point out which figure he considers most imperfectly made. If he should select the 1, say, "You had better practise making that figure. Oblige me by multiplying the row by nine." When he does so the result will be
Then say, "After this practice you will be able to make better ones in future."