Four lots of dots
The picture shows 16 dots drawn in an evenly spaced 4-by-4 square array. How many squares can be made using four of the dots as the vertices? In fact there are 20, including the one shown.
I have now drawn another square array of the same type but with a larger number of dots. This time the number of squares possible is four times the number of dots.
How many dots have I drawn?
WIN £15 will be awarded to the sender of the first correct answer opened on Wednesday 12 December. The Editor’s decision is final. Please send entries to Enigma 1723, Â鶹´«Ã½, Lacon House, 84 Theobald’s Road, London WC1X 8NS, or to enigma@newscientist.com (please include your postal address).
Answer to 1717 On the beach: 10 minutes
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The winner Howard Bobry of Edmonds, Washington, US
