Question 5
Eighteen disks hang by threads from a rod in six columns of three. Reading each column from top to bottom the numbers are: 3-7-8, 6-9-4, 2-4-5, 7-6-1, 8-1-3, 2-9-5 (the top disk of each column hangs from the rod; each lower disk hangs from the one above). Cut threads so that one set of numbers drops and nine disks still hang from the rod showing the numbers 1 to 9 once each. What is the least number of threads that must be cut? Answer with a natural number.
