Just ladders
I have designed a snakes and ladders board. It is a 10-by-10 grid numbered 1 to 10 from left to right in the bottom row, then 11 to 20 from right to left in the next row, 21 to 30 from left to right in the next and so on, ending at 100 in the top left-hand corner.
Each ladder is a thin straight line from the centre of one square up to the centre of another square. None goes up vertically and no two of the ladders touch or cross each other. If a ladder started, say, in the second row up and went to the seventh row up then it would completely jump over four rows. In fact, my seven ladders all jump over at least one row and they all jump different numbers of rows from each other.
The numbers at the bottom of the ladders are all perfect squares larger than 1, and the numbers at the top of the ladders are all prime numbers. List the starts and finishes of all the ladders (in the form 4-a, 9-b, etc).
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WIN 拢15 will be awarded to the sender of the first correct answer opened on Wednesday 16 December. The Editor鈥檚 decision is final. Please send entries to Enigma 1571, New Scientist, Lacon House, 84 Theobald鈥檚 Road, London WC1X 8NS, or to enigma@newscientist.com (please include your postal address).
Answer to 1565 It鈥檚 a rollover: The minimum number of moves are 10, 9, 8, 7, 8 and 7
The winner Tom Marlow of Saffron Walden, Essex, UK