Ask a person to think of a number; then tell him to subtract 1 from that number; now tell him to multiply the remainder by 2; then request him again to subtract 1, and add to the remainder the number he first thought of, and to inform you of the total. When he has done this, you must mentally add three to that total, and then divide it by 3, and the quotient will be the number first thought of. This is an excellent arithmetical pastime, examples of which we give below :

10

15

18

23

1

1

1

1

___

___

9

14

17

22

2

2

2

2

__

____

18

28

34

44

1

1

1

1

____

___

___

____

17

27

33

43

10

15

18

23

___

___

___

27

42

51

66

3

3

3

3

3)30

3)45

3)54

3)69

___

____

____

. ,

10

15

18

23