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Pigeonhole Principle Worksheet for Maths Olympiad

Maths Olympiad topic 23: Pigeonhole and worst case · Set 82 · 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 10 black balls and 4 yellow 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) 2 (c) 14 (d) 5
  2. 2.A drawer has lots of 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) 5 (c) 3 (d) 4
  3. 3.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) 13 (b) 25 (c) 24 (d) 12
  4. 4.A bag has 3 yellow balls and 3 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 4 (b) 5 (c) 2 (d) 6
  5. 5.A drawer has lots of 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) 6 (b) 2 (c) 4 (d) 3
  6. 6.A drawer has lots of 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) 2 (b) 3 (c) 4 (d) 5
  7. 7.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) 25 (c) 36 (d) 24
  8. 8.A bag has 4 green balls and 9 blue balls. Without looking, how many balls must you take out to be sure of getting 2 blue balls? (a) 2 (b) 5 (c) 13 (d) 6
  9. 9.A bag has 4 yellow 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) 10 (b) 13 (c) 2 (d) 5
  10. 10.A drawer has lots of green, 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) 6
  11. 11.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
  12. 12.A bag has 7 green balls and 4 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) 2 (d) 8
  13. 13.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) 15 (c) 14 (d) 7
  14. 14.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) 49 (b) 48 (c) 36 (d) 37
  15. 15.A bag has 4 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) 13 (b) 2 (c) 16 (d) 5
  16. 16.A bag has 10 green balls and 5 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) 6 (d) 11
  17. 17.A bag has 12 green balls and 7 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) 13 (d) 19
  18. 18.A bag has 11 red balls and 6 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 17 (b) 12 (c) 13 (d) 2
  19. 19.A drawer has lots of yellow, 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) 6 (b) 3 (c) 2 (d) 4
  20. 20.A bag has 8 red balls and 12 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 20 (b) 2 (c) 10 (d) 9
  21. 21.A bag has 5 green balls and 4 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 7 (b) 2 (c) 6 (d) 9
  22. 22.A bag has 8 green balls and 8 blue balls. Without looking, how many balls must you take out to be sure of getting 2 blue balls? (a) 2 (b) 10 (c) 16 (d) 9
  23. 23.A bag has 10 black balls and 8 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 2 (b) 18 (c) 12 (d) 11
  24. 24.A bag has 4 red 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) 7 (b) 2 (c) 4 (d) 5
  25. 25.A bag has 3 red 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) 5 (c) 15 (d) 2
  26. 26.A bag has 3 green 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) 2 (b) 10 (c) 12 (d) 4
  27. 27.A drawer has lots of white, black, 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) 8 (b) 4 (c) 2 (d) 5
  28. 28.A bag has 12 green balls and 7 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 2 (b) 13 (c) 19 (d) 14
  29. 29.A bag has 12 blue balls and 9 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 14 (b) 13 (c) 2 (d) 21
  30. 30.A bag has 8 green 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) 7 (b) 2 (c) 14 (d) 9
  31. 31.A drawer has lots of green, white, 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) 4 (c) 5 (d) 8
  32. 32.A drawer has lots of black, white, 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) 5 (b) 6 (c) 2 (d) 10
  33. 33.A bag has 7 red balls and 4 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) 11 (c) 8 (d) 5
  34. 34.A bag has 8 black balls and 7 blue 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) 9 (d) 15
  35. 35.A bag has 10 green 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) 16 (c) 11 (d) 12
  36. 36.A bag has 11 black balls and 11 blue balls. Without looking, how many balls must you take out to be sure of getting 2 blue balls? (a) 22 (b) 13 (c) 12 (d) 2
  37. 37.A bag has 4 green balls and 5 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) 6 (c) 5 (d) 2
  38. 38.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) 8 (b) 2 (c) 7 (d) 10
  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 day of the week? (a) 28 (b) 22 (c) 21 (d) 29
  40. 40.A bag has 10 green balls and 6 yellow balls. Without looking, how many balls must you take out to be sure of getting 2 yellow balls? (a) 11 (b) 12 (c) 16 (d) 2

Answer key

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

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