Sunday Times Teaser 2609 – String Art
by Andrew Skidmore
Published: 23 September 2012 (link)
Callum knocked some tacks onto the edge of a circular board. He then used pieces of string to make all possible straight-line connections between pairs of tacks. The tacks were arranged so that no three pieces of string crossed at the same point inside the circle. If he had used six tacks this would have required fifteen pieces of string and it would have created fifteen crossing points inside the circle. But he used more than six and in his case the number of crossing points inside the circle was a single-digit multiple of the number of pieces of string used.
How many tacks did he use?