Unit 9 · Speed and Time Calculation
Sally's Sandwich Rules — What Goes on Top?
Lesson 8 of 9
About this lesson
Four sandwich ingredients — chicken, mayonnaise, cheese, lettuce — must be stacked in four layers. Three rules constrain the order: lettuce can't touch mayo, cheese can't touch lettuce, and mayo goes directly above chicken. The direct-adjacency rule (mayo on top of chicken) is the strongest starting point — use it to anchor the pair and then test where the pair can go.
Chapters
Question
Chris is making a sandwich for his younger sister Sally. Sally knows that Chris likes puzzles, so she challenges him to arrange the four ingredients in her sandwich in a specific way. She tells him that the lettuce must not touch the mayonnaise, the cheese must not touch the lettuce, and the mayonnaise must go directly on top of the chicken.
If Chris makes the sandwich following Sally's rules, which ingredient will be on top?
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