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Pigeonhole Principle Worksheet for Maths Olympiad

Maths Olympiad topic 23: Pigeonhole and worst case · Set 142 · 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 3 red balls and 7 green balls. Without looking, how many balls must you take out to be sure of getting 2 green balls? (a) 4 (b) 2 (c) 10 (d) 5
  2. 2.A drawer has lots of white, red, 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) 10 (b) 5 (c) 6 (d) 2
  3. 3.A bag has 12 yellow 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) 5 (b) 16 (c) 2 (d) 13
  4. 4.A bag has 12 white 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) 2 (b) 9 (c) 20 (d) 13
  5. 5.A bag has 3 white balls and 4 green balls. Without looking, how many balls must you take out to be sure of getting 2 green balls? (a) 7 (b) 5 (c) 4 (d) 2
  6. 6.A bag has 4 green balls and 5 blue 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) 9 (c) 5 (d) 2
  7. 7.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) 22 (c) 29 (d) 28
  8. 8.A drawer has lots of black, yellow, 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) 8 (b) 2 (c) 5 (d) 4
  9. 9.A drawer has lots of white, 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) 6 (b) 4 (c) 2 (d) 3
  10. 10.A bag has 9 green balls and 11 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) 10 (c) 2 (d) 20
  11. 11.A bag has 9 green balls and 3 yellow balls. Without looking, how many balls must you take out to be sure of getting 2 yellow balls? (a) 12 (b) 2 (c) 10 (d) 11
  12. 12.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) 6 (b) 3 (c) 4 (d) 2
  13. 13.A bag has 8 black balls and 12 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 2 (b) 20 (c) 9 (d) 10
  14. 14.A drawer has lots of white, yellow, blue, 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) 6 (c) 5 (d) 10
  15. 15.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) 25 (b) 37 (c) 24 (d) 36
  16. 16.A bag has 6 green balls and 12 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 18 (b) 7 (c) 8 (d) 2
  17. 17.A bag has 4 red 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) 2 (b) 5 (c) 13 (d) 10
  18. 18.A drawer has lots of 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) 4 (b) 2 (c) 3 (d) 6
  19. 19.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
  20. 20.A drawer has lots of green 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) 2 (c) 5 (d) 4
  21. 21.A bag has 8 blue balls and 9 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) 10 (c) 17 (d) 2
  22. 22.A bag has 12 black balls and 13 blue 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
  23. 23.A drawer has lots of green, 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) 2 (b) 3 (c) 4 (d) 6
  24. 24.A drawer has lots of green, 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) 2 (b) 8 (c) 5 (d) 4
  25. 25.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) 22 (b) 21 (c) 14 (d) 15
  26. 26.A bag has 4 white balls and 12 yellow balls. Without looking, how many balls must you take out to be sure of getting 2 yellow balls? (a) 2 (b) 5 (c) 6 (d) 16
  27. 27.A bag has 7 yellow balls and 11 black 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) 18 (d) 2
  28. 28.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) 37 (d) 49
  29. 29.A bag has 12 blue balls and 6 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 18 (b) 2 (c) 13 (d) 14
  30. 30.A drawer has lots of blue, black, 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) 2 (b) 4 (c) 5 (d) 8
  31. 31.A bag has 12 black balls and 11 green balls. Without looking, how many balls must you take out to be sure of getting 2 green balls? (a) 2 (b) 14 (c) 23 (d) 13
  32. 32.A bag has 12 yellow 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) 19 (b) 2 (c) 13 (d) 8
  33. 33.A drawer has lots of green, 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) 3 (c) 6 (d) 2
  34. 34.A bag has 3 blue balls and 8 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 2 (b) 11 (c) 5 (d) 4
  35. 35.A bag has 3 green 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) 10 (c) 4 (d) 12
  36. 36.A bag has 12 red 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) 2 (b) 6 (c) 17 (d) 13
  37. 37.A drawer has lots of yellow, 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) 4 (d) 3
  38. 38.A bag has 6 black balls and 9 green balls. Without looking, how many balls must you take out to be sure of getting 2 green balls? (a) 7 (b) 8 (c) 15 (d) 2
  39. 39.A bag has 5 blue balls and 11 green balls. Without looking, how many balls must you take out to be sure of getting 2 green balls? (a) 6 (b) 7 (c) 16 (d) 2
  40. 40.A bag has 4 yellow balls and 12 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 2 (b) 5 (c) 16 (d) 6

Answer key

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

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