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Pigeonhole Principle Worksheet for Maths Olympiad

Maths Olympiad topic 23: Pigeonhole and worst case · Set 646 · 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 green, blue, 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) 8 (d) 4
  2. 2.A bag has 8 black balls and 11 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 19 (b) 10 (c) 2 (d) 9
  3. 3.A drawer has lots of black, red, green, yellow 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) 10 (b) 2 (c) 5 (d) 6
  4. 4.A bag has 4 black balls and 11 green 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) 15 (c) 5 (d) 2
  5. 5.A bag has 4 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) 9 (c) 12 (d) 5
  6. 6.A bag has 7 red balls and 6 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 8 (b) 2 (c) 9 (d) 13
  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 day of the week? (a) 14 (b) 22 (c) 21 (d) 15
  8. 8.A bag has 8 blue balls and 10 yellow balls. Without looking, how many balls must you take out to be sure of getting 2 yellow balls? (a) 18 (b) 2 (c) 10 (d) 9
  9. 9.A drawer has lots of green, white, red, 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) 2 (b) 6 (c) 10 (d) 5
  10. 10.A bag has 9 yellow balls and 12 black balls. Without looking, how many balls must you take out to be sure of getting 2 black balls? (a) 21 (b) 11 (c) 10 (d) 2
  11. 11.A drawer has lots of blue, green, black, 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) 6 (b) 10 (c) 5 (d) 2
  12. 12.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) 2 (b) 11 (c) 12 (d) 15
  13. 13.A bag has 12 black 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) 6 (b) 13 (c) 2 (d) 17
  14. 14.A bag has 4 green balls and 7 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 6 (b) 11 (c) 2 (d) 5
  15. 15.A drawer has lots of 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) 2 (b) 3 (c) 4 (d) 5
  16. 16.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) 4 (c) 2 (d) 5
  17. 17.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) 12 (c) 24 (d) 25
  18. 18.A bag has 5 red balls and 12 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) 13 (c) 6 (d) 17
  19. 19.A drawer has lots of blue, 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) 4 (b) 3 (c) 6 (d) 2
  20. 20.A bag has 12 green balls and 6 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) 18 (c) 13 (d) 7
  21. 21.A drawer has lots of green, red, white, 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) 5 (b) 2 (c) 6 (d) 10
  22. 22.A drawer has lots of red, blue, 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) 4 (b) 5 (c) 2 (d) 8
  23. 23.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) 2 (b) 5 (c) 3 (d) 4
  24. 24.A bag has 3 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) 12 (b) 4 (c) 10 (d) 2
  25. 25.A drawer has lots of green, 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) 4 (b) 8 (c) 2 (d) 5
  26. 26.A bag has 10 white balls and 7 green balls. Without looking, how many balls must you take out to be sure of getting 2 green balls? (a) 12 (b) 2 (c) 11 (d) 17
  27. 27.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) 21 (d) 22
  28. 28.A drawer has lots of blue 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
  29. 29.A drawer has lots of red, 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) 4 (b) 2 (c) 3 (d) 6
  30. 30.A bag has 3 red balls and 9 black 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) 2 (c) 12 (d) 4
  31. 31.A drawer has lots of red, blue, white, 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) 2 (b) 5 (c) 10 (d) 6
  32. 32.A drawer has lots of yellow, white, 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) 5 (b) 2 (c) 4 (d) 8
  33. 33.A bag has 10 green 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) 2 (b) 5 (c) 14 (d) 11
  34. 34.A bag has 12 red balls and 3 green 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) 2 (c) 13 (d) 4
  35. 35.A drawer has lots of black, white, 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) 8 (b) 5 (c) 4 (d) 2
  36. 36.A bag has 12 yellow balls and 8 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 14 (b) 13 (c) 20 (d) 2
  37. 37.A bag has 3 white 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) 4 (b) 8 (c) 2 (d) 6
  38. 38.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) 2 (c) 5 (d) 3
  39. 39.A bag has 7 black balls and 4 white balls. Without looking, how many balls must you take out to be sure of getting 2 white balls? (a) 11 (b) 9 (c) 2 (d) 8
  40. 40.A bag has 3 yellow balls and 7 red balls. Without looking, how many balls must you take out to be sure of getting 2 red balls? (a) 2 (b) 5 (c) 4 (d) 10

Answer key

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

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