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Mary Robinson Eamonn Farrell
brainteaser

Maths Week: Your Thursday puzzle

Day 6 – and this is a tough one. Follow the instructions closely.

WE’RE GETTING TO the end of Math’s Week… so the puzzles are getting harder. Don’t be too scared though – there’s a little magic in this one. 

DOES THE IRISH president have special powers? Well, you might be surprised that the very name of an Irish President has special powers.

You can harness these special powers to astound your friends using an ordinary pack of cards. You don’t even need to play with a full deck—but you will need a friend who can name any of the nine people who have held the office of Uachtarán na hÉireann.

Take 11 cards from an ordinary pack, setting the rest aside.

Ask a friend to mix up this small packet, face down so nobody sees any card faces. Take the cards back and tap them on the edge of the table to even them up, asking for any Irish president in history to be named.

Suppose Mary Robinson is selected:

Hold the packet in one hand, and with the other hand count out 8 cards to the table, one on top of the other, as you spell out R-O-B-I-N-S-O-N, thereby reversing the order of those cards.

Say “Robinson” out loud as you drop the remaining cards (as a unit) on top of the 8 on the table. Now do the same thing using M-A-R-Y, counting out and reversing just 4 cards, and say “Mary” out loud before dropping the rest on top as a unit.

Finally, do the process one last time again using Robinson again.

At the end, say, “You mixed the cards, you named a president, all I did was spell out the words while counting cards to the table, and drop what was left on top, three times, thereby further randomizing the cards.”

Your friend nods in agreement. Continue, “But I can see that the top card is now the 3 of Spades. I guess even ordinary citizens can channel presidential powers!”

The friend then turns over the top card and is astonished to find that you are correct.

This can be repeated as often as you wish, using any 11 cards and any Irish president’s name. Each time, you will know what ends up on the top after the third spelling. It even works if, instead of spelling “Robinson, Mary, Robinson” as above, you spell “Mary, Robinson, Mary.”

How is this possible?

As it is a maths puzzle, I should perhaps reveal that while the friend thinks of a president, you peek at the bottom card while tapping the cards on the table to even them up. 

Come back tomorrow for the answers to today’s puzzle. 

Wednesday’s puzzle: the answer

1.

(A) 5
(B) 13
(C) 6
(D) 14

2.

(A) *2+3 4
(B) +*2 3 *4 5
(C) +-+ 6 5 4 3
(D) /*6 5 + 7 3

Maths Week Ireland is coordinated by SETU with partners across the island of Ireland. This year over 400,000, north and south will take part and these puzzles give you a chance to participate. 

The Maths Week puzzles this year are presented by Colm Mulcahy, professor emeritus of Mathematics at Spelman College, USA, and adjunct professor with Calmast at South East Technological University. Colm is chairperson of the Martin Gardner Foundation USA, and the curator of the website mathsireland.ie

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