Henry Ernest Dudeney/Modern Puzzles/100 - Odds and Evens

Modern Puzzles by Henry Ernest Dudeney: $100$

Odds and Evens

Here is a little parlour trick, the explanation of which is quite easy.

Ask a friend to take an even number of coins in one hand and an odd number in the other.

You then undertake to tell him which hand holds the odd and which the even.

Tell him to multiply the number in the right hand by $7$ and the number in the left by $6$,

add the two products together, and tell you the result.

You can then immediately give him the required answer.

How are you to do it?

In practice, after he had given me the result, I should say,

"Let us add your age, now deduct the day of the month," and so on --

a lot of absurd apparent calculations merely devised for his mystification,

for you already know the answer.

Sources

• 1926: Henry Ernest Dudeney: Modern Puzzles ... (previous) ... (next): Arithmetical and Algebraical Problems: Various Arithmetical and Algebraical Problems: $100$. -- Odds and Evens