A divisibility problem

How many 4-digit positive integers which comprise of at most 2 distinct non-zero digits, are divisible by 11?

Note/Hint: A number can be in the form of a a a a \overline {aaaa} or b b b b \overline {bbbb} .


The answer is 153.

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1 solution

There are two groups of four-digit numbers divisible by 11 11 that fulfill the conditions of values of a a and b b . The first group is consisted of palindromes. Palindromes are numbers that are read the same from left to right and from right to left. Here is a proof of this claim, taking for example a number with a form of a b b a \overline{abba}

a b b a \overline {abba} = = 1000 × a + 100 × b + 10 × b + a 1000\times a + 100\times b + 10\times b + a = = 1001 a + 110 b 1001a + 110b

Because 11 1001 11|1001 and 11 110 11|110 = > => 11 a b b a 11|\overline{abba}

Palindromes come in the following forms:

  • a a a a \overline{aaaa}
  • b b b b \overline{bbbb}
  • a b b a \overline{abba}
  • b a a b \overline{baab}

The other group consists of numbers that are in forms of:

  • a a b b \overline{aabb}
  • b b a a \overline{bbaa}

And here is the proof for this claim, taking for example a number with a form of a a b b \overline{aabb} :

a a b b \overline {aabb} = = 1000 × a + 100 × a + 10 × b + b 1000\times a + 100\times a + 10\times b + b = = 1100 a + 11 b 1100a + 11b

Because 11 1100 11|1100 and 11 11 11|11 = > => 11 a a b b 11|\overline{aabb}

There is a total of 153 153 numbers that fulfill all of the requirements above.

For those interested, the numbers are as follows:

  1. 1111
  2. 1122
  3. 1133
  4. 1144
  5. 1155
  6. 1166
  7. 1177
  8. 1188
  9. 1199
  10. 2222
  11. 2211
  12. 2233
  13. 2244
  14. 2255
  15. 2266
  16. 2277
  17. 2288
  18. 2299
  19. 3333
  20. 3311
  21. 3322
  22. 3344
  23. 3355
  24. 3366
  25. 3377
  26. 3388
  27. 3399
  28. 4411
  29. 4422
  30. 4433
  31. 4444
  32. 4455
  33. 4466
  34. 4477
  35. 4488
  36. 4499
  37. 5511
  38. 5522
  39. 5533
  40. 5544
  41. 5555
  42. 5566
  43. 5577
  44. 5588
  45. 5599
  46. 6611
  47. 6622
  48. 6633
  49. 6644
  50. 6655
  51. 6666
  52. 6677
  53. 6688
  54. 6699
  55. 7711
  56. 7722
  57. 7733
  58. 7744
  59. 7755
  60. 7766
  61. 7777
  62. 7788
  63. 7799
  64. 8811
  65. 8822
  66. 8833
  67. 8844
  68. 8855
  69. 8866
  70. 8877
  71. 8888
  72. 8899
  73. 9911
  74. 9922
  75. 9933
  76. 9944
  77. 9955
  78. 9966
  79. 9977
  80. 9988
  81. 9999
  82. 1221
  83. 1331
  84. 1441
  85. 1551
  86. 1661
  87. 1771
  88. 1881
  89. 1991
  90. 2112
  91. 2332
  92. 2442
  93. 2552
  94. 2662
  95. 2772
  96. 2882
  97. 2992
  98. 3113
  99. 3223
  100. 3443
  101. 3553
  102. 3663
  103. 3773
  104. 3883
  105. 3993
  106. 4114
  107. 4224
  108. 4334
  109. 4554
  110. 4664
  111. 4774
  112. 4884
  113. 4994
  114. 5115
  115. 5225
  116. 5335
  117. 5445
  118. 5665
  119. 5775
  120. 5885
  121. 5995
  122. 6116
  123. 6226
  124. 6336
  125. 6446
  126. 6556
  127. 6776
  128. 6886
  129. 6996
  130. 7117
  131. 7227
  132. 7337
  133. 7447
  134. 7557
  135. 7667
  136. 7887
  137. 7997
  138. 8118
  139. 8228
  140. 8338
  141. 8448
  142. 8558
  143. 8668
  144. 8778
  145. 8998
  146. 9119
  147. 9229
  148. 9339
  149. 9449
  150. 9559
  151. 9669
  152. 9779
  153. 9889

Djordje Veljkovic - 4 years, 4 months ago

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This leaves out numbers with zeros in them.

Geoff Pilling - 4 years, 4 months ago

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Thanks. I removed "non-zero" when I edited the problem for clarity.

Those who answered 171 have been marked correct.

Calvin Lin Staff - 4 years, 4 months ago

This solution shows that these 2 groups satisfy the conditions in the problem, meaning that this condition is sufficient.

However, it does not show that these 2 groups are the only 2 groups, meaning that we haven't shown this condition is necessary. We need to explain why numbers of the form a a a b , a a b a \overline{aaab}, \overline{aaba} etc cannot be multiples of 11, for any values of a a and b b .

Calvin Lin Staff - 4 years, 4 months ago

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