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Pigeonhole Principle Worksheet for Maths Olympiad

Maths Olympiad topic 23: Pigeonhole and worst case · Set 567 · 40 questions
TalentJR

How to solve it (Think of the unluckiest case): Imagine the unluckiest draws: how many can you take and still NOT have what you want? Add 1 to that number — the next one must give it. Pigeonhole: with n boxes (colours, months, days), n × (k − 1) + 1 things make sure some box has at least k.

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  1. 1.A drawer has lots of yellow, red, white and blue socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 4 (b) 8 (c) 2 (d) 5
  2. 2.A drawer has lots of red, blue, white, green and black socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 2 (b) 5 (c) 10 (d) 6
  3. 3.A drawer has lots of yellow, black and green socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 2 (b) 6 (c) 4 (d) 3
  4. 4.A bag has 11 red balls and 8 blue balls. Without looking, how many balls must you take out to be sure of getting 2 blue balls? (a) 12 (b) 19 (c) 13 (d) 2
  5. 5.A drawer has lots of red, yellow and blue socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 3 (b) 2 (c) 4 (d) 6
  6. 6.A bag has 7 blue balls and 10 yellow balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 8 (b) 2 (c) 17 (d) 11
  7. 7.A bag has 11 red balls and 7 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 12 (b) 13 (c) 2 (d) 18
  8. 8.What is the smallest number of children in a group that makes sure at least three of them were born in the same day of the week? (a) 21 (b) 15 (c) 22 (d) 14
  9. 9.A drawer has lots of green and white socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 3 (b) 2 (c) 5 (d) 4
  10. 10.A drawer has lots of yellow and white socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 5 (b) 3 (c) 4 (d) 2
  11. 11.A drawer has lots of black and green socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 4 (b) 2 (c) 3 (d) 5
  12. 12.A bag has 11 red balls and 4 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 2 (b) 13 (c) 15 (d) 12
  13. 13.A bag has 9 blue balls and 10 green balls. Without looking, how many balls must you take out to be sure of getting 2 green balls? (a) 10 (b) 19 (c) 11 (d) 2
  14. 14.A drawer has lots of red, yellow, black and blue socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 8 (b) 5 (c) 2 (d) 4
  15. 15.A drawer has lots of black, red, green, blue and white socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 2 (b) 6 (c) 5 (d) 10
  16. 16.A drawer has lots of white and blue socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 4 (b) 2 (c) 5 (d) 3
  17. 17.What is the smallest number of children in a group that makes sure at least four of them were born in the same month? (a) 48 (b) 36 (c) 49 (d) 37
  18. 18.A drawer has lots of green, blue, red and black socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 5 (b) 4 (c) 8 (d) 2
  19. 19.A drawer has lots of green, white, blue, yellow and black socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 2 (b) 5 (c) 10 (d) 6
  20. 20.A bag has 5 black balls and 11 green balls. Without looking, how many balls must you take out to be sure of getting 2 green balls? (a) 6 (b) 16 (c) 7 (d) 2
  21. 21.What is the smallest number of children in a group that makes sure at least two of them were born in the same month? (a) 25 (b) 13 (c) 24 (d) 12
  22. 22.A drawer has lots of black and red socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 2 (b) 5 (c) 4 (d) 3
  23. 23.A bag has 4 green balls and 7 black balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 5 (b) 11 (c) 8 (d) 2
  24. 24.A drawer has lots of black, yellow, red and white socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 8 (b) 4 (c) 5 (d) 2
  25. 25.A bag has 9 red balls and 7 black balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 8 (b) 2 (c) 16 (d) 10
  26. 26.A drawer has lots of red, white, blue, yellow and green socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 2 (b) 10 (c) 5 (d) 6
  27. 27.A bag has 6 red balls and 10 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 16 (b) 2 (c) 7 (d) 8
  28. 28.A drawer has lots of blue and red socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 5 (b) 2 (c) 4 (d) 3
  29. 29.A bag has 3 green balls and 10 black balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 2 (b) 13 (c) 11 (d) 4
  30. 30.A bag has 6 black balls and 4 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 10 (b) 7 (c) 2 (d) 8
  31. 31.A bag has 5 blue balls and 7 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 2 (b) 6 (c) 12 (d) 7
  32. 32.A bag has 8 black balls and 9 green balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 17 (b) 9 (c) 2 (d) 10
  33. 33.What is the smallest number of children in a group that makes sure at least two of them were born in the same day of the week? (a) 14 (b) 7 (c) 15 (d) 8
  34. 34.A bag has 6 red balls and 6 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 8 (b) 12 (c) 7 (d) 2
  35. 35.A drawer has lots of blue, green, black and yellow socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 8 (b) 2 (c) 5 (d) 4
  36. 36.A bag has 5 red balls and 10 blue balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 2 (b) 15 (c) 11 (d) 6
  37. 37.A bag has 7 red balls and 9 yellow balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 8 (b) 16 (c) 2 (d) 10
  38. 38.A bag has 10 black balls and 7 green balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 17 (b) 11 (c) 8 (d) 2
  39. 39.A bag has 7 yellow balls and 3 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 8 (b) 10 (c) 9 (d) 2
  40. 40.A drawer has lots of white and yellow socks, all mixed up. In the dark, how many socks must you take out to be sure of getting a matching pair? (a) 5 (b) 4 (c) 3 (d) 2

Answer key

  1. 5
  2. 6
  3. 4
  4. 13
  5. 4
  6. 11
  7. 13
  8. 15
  9. 3
  10. 3
  11. 3
  12. 13
  13. 11
  14. 5
  15. 6
  16. 3
  17. 37
  18. 5
  19. 6
  20. 7
  21. 13
  22. 3
  23. 8
  24. 5
  25. 10
  26. 6
  27. 8
  28. 3
  29. 11
  30. 8
  31. 7
  32. 10
  33. 8
  34. 8
  35. 5
  36. 11
  37. 10
  38. 11
  39. 9
  40. 3

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