This section is from the book "Facts Worth Knowing", by Robert Kemp Philip. Also available from Amazon: Inquire Within for Anything You Want to Know.

A very pleasing way to arrive at an arithmetical sum, without the use of either 6late or pencil, is to ask a person to think of a figure, then to double it, then add a certain figure to it, now halve the whole sum, and finally to subtract from that the figure first thought of. You are then to tell the thinker what is the remainder.

The key to this lock of figures is, that half of whatever sum you require to be added during the working- of the sum is the remainder. In the example given, five is the half of ten, the number requested to be added. Any amount may be added, but the operation is simplified by giving only even numbers, as they will divide without fractions.

Example.

Think of.......... . 7

Double it...........= 14

Add to it............. 10

Halve it............ 1/2)24

Which will leave..... 12

Subtract the number thought of.......... 7

The remainder will be.. 5

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