Class 4
Chapter 2: Hide and Seek
When we have many possibilities, each clue lets us cross some out. The answer is whatever is left after all the clues are used. This careful reasoning is called logical elimination.
Learning outcomes
- Use clues to narrow down possibilities
- Apply logical elimination to puzzles
Activities
- Number-clue 'guess who' game
Worked examples
Read through these first, then try the practice below.
Example 1 — Find the number
I am thinking of a number from 1 to 10. It is even. It is greater than 6. It is not 10. What is it?
Solution: From 1-10, even numbers are 2, 4, 6, 8, 10. 'Greater than 6' leaves 8 and 10. 'Not 10' leaves 8.
Example 2 — Cross them out
Three friends are 9, 10 and 11 years old. Aru is not 11. Riya is older than Aru. Who is 10?
Solution: Aru is 9 or 10. Riya is older than Aru. If Aru is 10, Riya is 11. The third friend is 9. So Aru is 10? Yes — Aru is 10, Riya is 11, the third is 9.
Self-do practice
Question 1 of 3 · Score 0/0I am a number between 1 and 20. I am odd and I am a multiple of 5. Which number am I?
Sign in as a student in this class to save your progress.