Question 4
The beavers put 13 balls into a triangular box as illustrated below.
Figure label: “Hai quả bóng liền dưới” means “The two balls directly below”.
A ball is said to be in a “dangerous” position if it satisfies at least two of the following conditions:
- There is at least one empty space directly below that ball.
- There is at least one ball directly below that ball that is in a dangerous position.
How many balls in the box are not in a dangerous position?
Editorial clarification approved for grading: the original phrase “at least two” is preserved above. The intended rule in the official solution is that at least one of the two conditions suffices (OR). Answer using this intended rule. The literal AND reading is discussed separately in the solution.
Figure label: “Hai quả bóng liền dưới” means “The two balls directly below”.
A ball is said to be in a “dangerous” position if it satisfies at least two of the following conditions:
- There is at least one empty space directly below that ball.
- There is at least one ball directly below that ball that is in a dangerous position.
How many balls in the box are not in a dangerous position?
Editorial clarification approved for grading: the original phrase “at least two” is preserved above. The intended rule in the official solution is that at least one of the two conditions suffices (OR). Answer using this intended rule. The literal AND reading is discussed separately in the solution.