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Pigeonhole Principle Worksheet for Maths Olympiad

Maths Olympiad topic 23: Pigeonhole and worst case · Set 551 · 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.

NameDateTime takenScore ___ / 40
  1. 1.A bag has 5 green 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) 2 (c) 7 (d) 8
  2. 2.A bag has 12 blue balls and 3 green balls. Without looking, how many balls must you take out to be sure of getting 2 green balls? (a) 13 (b) 2 (c) 15 (d) 14
  3. 3.A bag has 11 blue balls and 9 green balls. Without looking, how many balls must you take out to be sure of getting 2 green balls? (a) 12 (b) 20 (c) 2 (d) 13
  4. 4.A drawer has lots of 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) 5 (b) 2 (c) 4 (d) 3
  5. 5.A drawer has lots of red, black 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) 4 (b) 6 (c) 2 (d) 3
  6. 6.A bag has 9 white balls and 5 green balls. Without looking, how many balls must you take out to be sure of getting 2 green balls? (a) 2 (b) 14 (c) 10 (d) 11
  7. 7.A drawer has lots of black, white, green, 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) 5 (c) 6 (d) 10
  8. 8.A bag has 9 black balls and 8 green 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) 17 (c) 10 (d) 9
  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 day of the week? (a) 22 (b) 29 (c) 21 (d) 28
  10. 10.A bag has 4 white balls and 4 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 2 (b) 6 (c) 8 (d) 5
  11. 11.A drawer has lots of red, 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) 3 (b) 6 (c) 2 (d) 4
  12. 12.A bag has 6 green balls and 11 white 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) 2 (c) 7 (d) 17
  13. 13.A drawer has lots of black, green, 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) 5 (b) 2 (c) 8 (d) 4
  14. 14.A drawer has lots of black, blue, 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) 5 (b) 8 (c) 2 (d) 4
  15. 15.A bag has 5 blue balls and 6 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) 2 (c) 7 (d) 11
  16. 16.A bag has 9 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) 19 (b) 11 (c) 10 (d) 2
  17. 17.A drawer has lots of yellow, green, black, 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) 2 (b) 10 (c) 6 (d) 5
  18. 18.A bag has 4 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) 2 (b) 15 (c) 12 (d) 5
  19. 19.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) 4 (b) 5 (c) 2 (d) 3
  20. 20.A bag has 9 red balls and 11 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) 20 (c) 10 (d) 12
  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 day of the week? (a) 15 (b) 7 (c) 8 (d) 14
  22. 22.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) 22 (c) 14 (d) 15
  23. 23.A bag has 12 green balls and 3 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 2 (b) 14 (c) 13 (d) 15
  24. 24.A drawer has lots of red 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) 2 (c) 4 (d) 3
  25. 25.A bag has 3 red balls and 6 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 2 (b) 9 (c) 5 (d) 4
  26. 26.A bag has 7 green balls and 8 black balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 15 (b) 8 (c) 2 (d) 9
  27. 27.A bag has 8 blue 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) 9 (c) 2 (d) 10
  28. 28.A bag has 8 yellow balls and 11 blue balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 19 (b) 2 (c) 9 (d) 12
  29. 29.A bag has 12 yellow balls and 13 black 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
  30. 30.A bag has 11 black balls and 9 green balls. Without looking, how many balls must you take out to be sure of getting 2 green balls? (a) 2 (b) 12 (c) 13 (d) 20
  31. 31.A bag has 11 white balls and 11 blue balls. Without looking, how many balls must you take out to be sure of getting 2 blue balls? (a) 13 (b) 12 (c) 2 (d) 22
  32. 32.A bag has 4 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) 4 (b) 2 (c) 5 (d) 7
  33. 33.A bag has 5 white balls and 6 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 11 (b) 6 (c) 2 (d) 7
  34. 34.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) 2 (b) 4 (c) 5 (d) 3
  35. 35.A drawer has lots of blue, white 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) 4 (b) 2 (c) 3 (d) 6
  36. 36.A drawer has lots of black, 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) 4 (b) 3 (c) 6 (d) 2
  37. 37.A bag has 3 white balls and 10 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 13 (b) 5 (c) 2 (d) 4
  38. 38.A bag has 5 yellow balls and 8 red balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 13 (b) 2 (c) 9 (d) 6
  39. 39.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) 49 (d) 48
  40. 40.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) 24 (b) 36 (c) 37 (d) 25

Answer key

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

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