Unit 12 · Multi-Step Conditional Reasoning
Alex, Ben, Carlos — Pets Difference Deduction
Lesson 1 of 16
This is a set-difference question: you need to find the pets Alex has that neither Ben nor Carlos has. The fastest method is to go through Alex’s list one by one and cross out any pet that appears on Ben’s OR Carlos’s list. What survives is the answer.
Set-difference membership questions (find what belongs to one person but not the others) appear regularly in OC TS and are reliable easy marks once you use a systematic check rather than trying to reason about all three lists at once in your head.
The examiner checks whether students can carefully compare three overlapping lists and correctly identify the items that are exclusive to one list — without missing an overlap or incorrectly excluding an item.
Three people each have a list of items (pets, hobbies, foods). You are asked which items on one person’s list do not appear on either of the other two lists. The answer always contains exactly two items.
Best approach: Write Alex’s list, then scan each item: does it appear in Ben’s list? Does it appear in Carlos’s list? If it appears in either, cross it out. The surviving items are the answer. Never try to hold all three lists in your head at the same time.
Question
Alex, Ben and Carlos all keep pets.
Alex has snakes, fish, lizards, rats and frogs.
Ben keeps cats, frogs and rats.
Carlos keeps fish, parrots, rats, grasshoppers and frogs.
Which pets does Alex keep that neither Ben nor Carlos keep?
Select an answer to see the explanation.
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