This is a fiendish puzzle which I first encountered at University. After a week of trying to solve it I gave up. I then challenged my brother to try it and was incensed to find out when I called him that he’d cracked it! I didn’t ask him for the solution and put it on the “back burner” for later but I never got round to solving it.
https://www.mathsisfun.com/pool_balls.html
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