The following puzzle is from the October 1961 issue of the Eureka journal (published by The Cambridge University Mathematical Society).
Arrange 15 balls, 3 of each of 5 colours, in the triangular array below, so that no two of the same colour lie in any line parallel to any side of the triangle.
O O O
O O O O
O O O O O
I found the phrase ‘3 of each of 5 colours’ a bit hard to understand. My question is, can anyone suggest a easier to understand wording of this phrase? Any other suggestions to make the question easier to understand would be appreciated. I hope to eventually share this puzzle with students in grade 9 or 10.