To Guess Any Number Thought of
With these thought-reading tricks may be put one or two arithmetical puzzles. Here is a way to find out the number that a person has thought of. Tell him to think of any number, odd or even. (Let us suppose that he thinks of 7.) Then tell him to double it (14), add 6 to it (20), halve it (10), and multiply it by 4 (40). Then ask him how many that makes. He will say 40. You divide this in your mind by 2 (20), subtract 6 (14), divide by 2 again (7), and astonish him by saying that the number of which he thought was 7.
To Guess Any Even Number Thought of
In this case you insist on the number chosen being an even number. Let us suppose it is 8. Tell him to multiply by 3 (24), halve it (12), multiply by 3 again (36), and then to tell you how many times 9 will go into the result. He will say 4. Double this in your mind and tell him that he thought of 8.
To Guess the Result of a Sum
Another trick. Tell the person to think of a number, to double it, add 6 to it, halve it and take away the number first thought of. When this has been done you tell him that 3 remains. If these directions are followed 3 must always remain. Let us take 7 and 1 as examples. Thus 7 doubled is 14; add 6 and it is 20; halved, it is 10; and if the number first thought of—7—is subtracted, 3 remains. Again, 1 doubled is 2; 6 added makes 8; 8 halved is 4, and 1 from 4 leaves 3.
A more bewildering puzzle is this. Tell as many persons as like to, to think of some number less than 1,000, in which the last figure is smaller than the first. Thus 998 might be thought of, but not 999, and not 347. The amount being chosen and written down, you tell each person to reverse the digits; so that the units come under the hundreds, the tens under the tens, and the hundreds under the units. Then tell them to subtract, to reverse again, and add; remarking to each one that you know what the answer will be. It will always be 1089. Let us suppose that three players choose numbers, one being 998, one 500, and one 321. Each sets them on paper, reverses the figures, and subtracts. Thus:—
| 998 | 500 | 321 |
| 899 | 005 | 123 |
| ———— | ———— | ———— |
| 099 | 495 | 198 |
The figures are then reversed and added. Thus:—
| 099 | 495 | 198 |
| 990 | 594 | 891 |
| ———— | ———— | ———— |
| 1089 | 1089 | 1089 |