You have 8 balls numbered 1, 3, 5, 7, 9, 11, 13 and 15. Select 3 balls and put them into the circles such that the sum of the numbers on them add up to 30.
Answer
Rotate ball 9 to make into a 6. Place the balls 6, 11 and 13 to get a sum of 30.
All the numbers are odd. There is no way to get an even number, 30, by summing three odd numbers together. At least one even number is required.