[ Union, Intersection, and Difference of Sets ]

In the first pencil case, there is a lilac pen, a green pencil, and a red eraser; in the second - a blue pen, a green pencil, and a yellow eraser; in the third - a lilac pen, an orange pencil, and a yellow eraser. The contents of these pencil cases follow this pattern: in any two of them, exactly one pair of items matches both in color and purpose. What should be in the fourth pencil case to maintain this pattern? (Each pencil case contains exactly three items: a pen, a pencil, and an eraser.)

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## Solution

In the fourth pencil case, there should be items that have already appeared in the first three pencil cases, but only once each.

## Answer

Blue pen, orange pencil, red eraser.