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Pigeonhole Principle Worksheet for Maths Olympiad

Maths Olympiad topic 23: Pigeonhole and worst case · Set 308 · 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 bag has 7 green balls and 10 blue balls. Without looking, how many balls must you take out to be sure of getting 2 blue balls? (a) 2 (b) 17 (c) 9 (d) 8
  2. 2.A bag has 11 white balls and 12 black balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 23 (b) 2 (c) 13 (d) 12
  3. 3.A drawer has lots of red, 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) 2 (b) 4 (c) 5 (d) 8
  4. 4.A drawer has lots of yellow, black, red, 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) 6 (c) 10 (d) 2
  5. 5.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) 8 (b) 14 (c) 15 (d) 7
  6. 6.A bag has 8 blue balls and 12 red 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) 20 (d) 13
  7. 7.A drawer has lots of black, white, blue, green 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) 10 (c) 5 (d) 6
  8. 8.A drawer has lots of yellow, 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) 8 (b) 5 (c) 2 (d) 4
  9. 9.A bag has 8 white balls and 6 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 14 (b) 10 (c) 2 (d) 9
  10. 10.A drawer has lots of white, red, 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) 8 (c) 5 (d) 2
  11. 11.A drawer has lots of 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) 8 (b) 2 (c) 4 (d) 5
  12. 12.A bag has 4 black 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) 5 (d) 2
  13. 13.What is the smallest number of children in a group that makes sure at least four of them were born in the same day of the week? (a) 28 (b) 29 (c) 22 (d) 21
  14. 14.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) 12 (c) 25 (d) 13
  15. 15.A drawer has lots of blue, black, red, white 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) 10 (b) 2 (c) 6 (d) 5
  16. 16.A bag has 11 green balls and 7 red balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 12 (b) 8 (c) 2 (d) 18
  17. 17.A bag has 11 blue balls and 4 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 12 (b) 2 (c) 13 (d) 15
  18. 18.A bag has 12 green balls and 11 red balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 12 (b) 13 (c) 2 (d) 23
  19. 19.A drawer has lots of red, yellow, blue 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) 2 (c) 8 (d) 4
  20. 20.A drawer has lots of red, yellow, white, 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) 10 (b) 2 (c) 5 (d) 6
  21. 21.A drawer has lots of yellow, black, green 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) 5 (c) 8 (d) 2
  22. 22.A drawer has lots of yellow 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) 4 (c) 2 (d) 3
  23. 23.A bag has 11 blue balls and 12 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 23 (b) 2 (c) 13 (d) 12
  24. 24.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) 4 (b) 2 (c) 3 (d) 5
  25. 25.A drawer has lots of 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) 3 (b) 2 (c) 4 (d) 5
  26. 26.A bag has 4 black balls and 10 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 2 (b) 5 (c) 6 (d) 14
  27. 27.A bag has 6 yellow balls and 8 blue balls. Without looking, how many balls must you take out to be sure of getting 2 blue balls? (a) 14 (b) 2 (c) 7 (d) 8
  28. 28.A bag has 12 yellow balls and 10 green balls. Without looking, how many balls must you take out to be sure of getting 2 green balls? (a) 14 (b) 13 (c) 2 (d) 22
  29. 29.A drawer has lots of blue 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) 4 (c) 5 (d) 3
  30. 30.A bag has 8 yellow balls and 11 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 2 (b) 19 (c) 9 (d) 10
  31. 31.A bag has 5 red 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) 5 (b) 2 (c) 9 (d) 6
  32. 32.A bag has 11 white balls and 10 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 21 (b) 13 (c) 12 (d) 2
  33. 33.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) 37 (b) 36 (c) 48 (d) 49
  34. 34.A bag has 11 black 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) 12 (b) 11 (c) 21 (d) 2
  35. 35.A drawer has lots of black, 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) 2 (b) 6 (c) 4 (d) 3
  36. 36.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) 36 (b) 25 (c) 37 (d) 24
  37. 37.A bag has 7 yellow balls and 10 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 8 (b) 9 (c) 17 (d) 2
  38. 38.A bag has 4 blue balls and 8 yellow balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 12 (b) 5 (c) 9 (d) 2
  39. 39.A bag has 10 green balls and 5 white balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 6 (b) 15 (c) 2 (d) 11
  40. 40.A bag has 12 white balls and 5 blue balls. Without looking, how many balls must you take out to be sure of getting 2 blue balls? (a) 13 (b) 17 (c) 14 (d) 2

Answer key

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

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