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Pigeonhole Principle Worksheet for Maths Olympiad

Maths Olympiad topic 23: Pigeonhole and worst case · Set 390 · 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 white, black, 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) 2 (b) 8 (c) 5 (d) 4
  2. 2.A bag has 6 white balls and 11 blue balls. Without looking, how many balls must you take out to be sure of getting 2 blue balls? (a) 17 (b) 7 (c) 2 (d) 8
  3. 3.A bag has 10 blue balls and 8 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 18 (b) 12 (c) 11 (d) 2
  4. 4.A drawer has lots of blue, green, 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) 8 (d) 4
  5. 5.A bag has 6 black balls and 4 blue balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 7 (b) 5 (c) 10 (d) 2
  6. 6.A bag has 11 yellow balls and 10 white 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) 21 (c) 12 (d) 11
  7. 7.A bag has 6 black balls and 3 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 8 (b) 9 (c) 7 (d) 2
  8. 8.A bag has 7 white balls and 12 blue balls. Without looking, how many balls must you take out to be sure of getting 2 blue balls? (a) 9 (b) 2 (c) 8 (d) 19
  9. 9.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
  10. 10.A bag has 7 green balls and 4 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 2 (b) 8 (c) 11 (d) 9
  11. 11.A bag has 6 blue balls and 10 red 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) 7 (c) 16 (d) 11
  12. 12.What is the smallest number of children in a group that makes sure at least three of them were born in the same month? (a) 37 (b) 36 (c) 25 (d) 24
  13. 13.A bag has 5 red balls and 12 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 7 (b) 2 (c) 17 (d) 6
  14. 14.A bag has 8 blue balls and 5 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 9 (b) 10 (c) 2 (d) 13
  15. 15.A bag has 3 blue balls and 8 white balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 9 (b) 4 (c) 11 (d) 2
  16. 16.A bag has 11 black balls and 6 yellow balls. Without looking, how many balls must you take out to be sure of getting 2 yellow balls? (a) 12 (b) 2 (c) 17 (d) 13
  17. 17.A bag has 12 white balls and 13 yellow balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 14 (b) 25 (c) 13 (d) 2
  18. 18.A bag has 7 red balls and 10 blue balls. Without looking, how many balls must you take out to be sure of getting 2 blue balls? (a) 9 (b) 17 (c) 2 (d) 8
  19. 19.A bag has 3 yellow balls and 12 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 4 (b) 2 (c) 5 (d) 15
  20. 20.A bag has 7 green balls and 7 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 2 (b) 9 (c) 14 (d) 8
  21. 21.A drawer has lots of green 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) 2 (b) 5 (c) 3 (d) 4
  22. 22.A drawer has lots of white, red, black, 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) 2 (b) 6 (c) 10 (d) 5
  23. 23.A drawer has lots of green, red, black, 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) 5 (b) 10 (c) 6 (d) 2
  24. 24.A bag has 8 red balls and 3 yellow balls. Without looking, how many balls must you take out to be sure of getting 2 yellow balls? (a) 11 (b) 9 (c) 10 (d) 2
  25. 25.A bag has 5 yellow balls and 3 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 6 (b) 7 (c) 8 (d) 2
  26. 26.A drawer has lots of green, yellow, blue, white 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) 10 (d) 6
  27. 27.A bag has 3 yellow balls and 4 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 7 (b) 2 (c) 5 (d) 4
  28. 28.A drawer has lots of red, blue, yellow, 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) 6 (b) 2 (c) 10 (d) 5
  29. 29.A bag has 12 red balls and 6 green balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 18 (b) 2 (c) 13 (d) 7
  30. 30.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) 7 (b) 15 (c) 14 (d) 8
  31. 31.A bag has 8 yellow balls and 10 red 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) 18 (c) 9 (d) 11
  32. 32.A bag has 7 white balls and 7 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 14 (b) 9 (c) 8 (d) 2
  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 month? (a) 24 (b) 13 (c) 25 (d) 12
  34. 34.A bag has 3 white balls and 6 blue balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 7 (b) 9 (c) 4 (d) 2
  35. 35.A drawer has lots of white, blue, red, green 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) 2 (b) 6 (c) 10 (d) 5
  36. 36.A bag has 3 black balls and 4 white balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 7 (b) 4 (c) 5 (d) 2
  37. 37.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) 5 (b) 2 (c) 3 (d) 4
  38. 38.A bag has 10 blue balls and 3 black balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 11 (b) 4 (c) 2 (d) 13
  39. 39.A bag has 6 yellow balls and 3 green balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 9 (b) 2 (c) 7 (d) 4
  40. 40.A drawer has lots of white, 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) 3 (b) 6 (c) 2 (d) 4

Answer key

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

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