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Programming Enigma Puzzles

12 September 2014

Posted by on **From New Scientist #2452, 19th June 2004** [link]

My young Australian nephew is getting good at arithmetic, and is gradually learning the names of a good number of animals. So I set him the following puzzle:

Think of a number, multiply it by 7, add 35, divide by 7, then take away the number you first thought of. Letting 1=A, 2=B, 3=C and so on, work out which letter your answer represents. Now think of an animal beginning with that letter.

In advance I had written “elephant” on a piece of card to amaze him. However, after thinking of a number he then performed the next four numerical steps in the wrong order, getting a positive whole number at each stage. He then worked out which letter his answer represented. However, he was unable to think of an animal beginning with that letter.

What number did he first think of, and what letter did he end up with?

**Note:** I am waiting for a phone line to be connected at my new house, so I only have sporadic access to the internet at the moment. The current estimate is that the line will be connected at the end of September 2014.

[enigma1294]

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This Python program runs in 38ms.

Solution:He started with 14 and ended up with the letter “Q”.There are only three options:

7 ⇒ “K”

14 ⇒ “Q”

21 ⇒ “W”

These can be achieved by:

× 7 → + 35 → –

m→ ÷ 7× 7 → –

m→ + 35 → ÷ 7Since the nephew is Australian we can assume that he knows of the Kangaroo/Koala/Kookaburra and Wombat/Wallaby, but perhaps not the Quokka/Quoll, leaving “Q” as the remaining option.