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Pigeonhole Principle Worksheet for Maths Olympiad

Maths Olympiad topic 23: Pigeonhole and worst case · Set 68 · 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, black, red 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) 4 (c) 2 (d) 5
  2. 2.A bag has 3 red balls and 6 yellow 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) 7 (d) 9
  3. 3.A bag has 12 red balls and 4 green balls. Without looking, how many balls must you take out to be sure of getting at least one ball of each colour? (a) 16 (b) 5 (c) 2 (d) 13
  4. 4.A bag has 12 green balls and 8 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 20 (b) 2 (c) 14 (d) 13
  5. 5.A drawer has lots of 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) 5 (b) 3 (c) 4 (d) 2
  6. 6.A drawer has lots of white, black, red 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) 2 (d) 5
  7. 7.A bag has 5 green balls and 9 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 6 (b) 7 (c) 14 (d) 2
  8. 8.A bag has 4 red balls and 12 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 5 (b) 2 (c) 6 (d) 16
  9. 9.A bag has 5 black 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) 14 (b) 6 (c) 10 (d) 2
  10. 10.A bag has 5 green balls and 5 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 2 (b) 10 (c) 6 (d) 7
  11. 11.A drawer has lots of red 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
  12. 12.A bag has 5 blue balls and 4 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 9 (b) 7 (c) 2 (d) 6
  13. 13.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) 24 (c) 37 (d) 25
  14. 14.A bag has 8 black 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) 2 (c) 9 (d) 10
  15. 15.A drawer has lots of blue, yellow, 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) 6 (b) 2 (c) 10 (d) 5
  16. 16.A bag has 3 white balls and 10 green 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) 13 (d) 11
  17. 17.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) 4 (b) 3 (c) 2 (d) 5
  18. 18.A bag has 10 yellow balls and 5 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 15 (b) 2 (c) 11 (d) 12
  19. 19.A drawer has lots of blue, 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) 4 (b) 6 (c) 3 (d) 2
  20. 20.A drawer has lots of green, white, yellow, 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) 10 (b) 5 (c) 6 (d) 2
  21. 21.A bag has 11 blue balls and 5 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 12 (b) 2 (c) 16 (d) 13
  22. 22.A drawer has lots of yellow, 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) 3 (b) 6 (c) 2 (d) 4
  23. 23.A bag has 7 yellow balls and 9 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) 8 (c) 10 (d) 16
  24. 24.A bag has 9 yellow balls and 9 blue balls. Without looking, how many balls must you take out to be sure of getting 2 blue balls? (a) 18 (b) 2 (c) 11 (d) 10
  25. 25.A bag has 12 green balls and 9 blue balls. Without looking, how many balls must you take out to be sure of getting 2 blue balls? (a) 21 (b) 13 (c) 2 (d) 14
  26. 26.A bag has 3 blue 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) 14 (d) 4
  27. 27.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) 12 (d) 24
  28. 28.A drawer has lots of 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) 3 (c) 2 (d) 5
  29. 29.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) 14 (b) 15 (c) 22 (d) 21
  30. 30.A bag has 9 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) 21 (b) 10 (c) 2 (d) 13
  31. 31.A drawer has lots of red, 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) 3 (c) 6 (d) 4
  32. 32.A drawer has lots of black, 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
  33. 33.A bag has 6 white 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) 16 (c) 7 (d) 8
  34. 34.A bag has 4 red balls and 7 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) 8 (c) 11 (d) 5
  35. 35.A bag has 9 red balls and 3 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 11 (b) 2 (c) 12 (d) 10
  36. 36.A bag has 4 blue balls and 5 yellow 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) 6 (d) 9
  37. 37.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) 21 (b) 29 (c) 28 (d) 22
  38. 38.A drawer has lots of red, 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
  39. 39.A bag has 12 red balls and 4 yellow balls. Without looking, how many balls must you take out to be sure of getting 2 yellow balls? (a) 13 (b) 2 (c) 14 (d) 16
  40. 40.A bag has 6 yellow balls and 7 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 8 (b) 13 (c) 7 (d) 2

Answer key

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

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