I got a question during an interview in a company and I still could not find out the solution. Kindly help me to solve this.


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    $\begingroup$ You mean "letter", not "alphabet". An alphabet is a set of letters. Not the first time I see this mistake. What translator did you use? May be worth sending some feedback. $\endgroup$ Mar 21, 2019 at 16:02
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    $\begingroup$ Was the question asked in English? Because these letters could be the first letters of a common set in another language eg( M,T,W,T,F, ?) would be S for Saturday. $\endgroup$ Mar 21, 2019 at 16:20
  • $\begingroup$ 8,1,8,12,21,? positions of the above letters in alphabet. i need to find the next position in the series $\endgroup$ Mar 21, 2019 at 16:41
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    $\begingroup$ what was the job? it might be relevant :) $\endgroup$
    – JMP
    Mar 21, 2019 at 17:26
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    $\begingroup$ @ArnaudMortier I think it's not so much a mistake as a dialect difference: in Indian English, "alphabet" means what "letter" means in most varieties of English. $\endgroup$
    – Gareth McCaughan
    Mar 21, 2019 at 18:17

2 Answers 2


I believe that the answer is


This is based on the fact that the first and last letters of each triplet is the same. For example, HAH, LUL, etc...


This is my random solution


The answer is:



Sequence is 8, 1, 8, 12, 21 - ignoring the first as red herring, the distance between each value is [7,4,9] (for example, 8 - 1 = 7, 12 - 8 = 4, etc.). The distance between those values shows a growing sequence of odd, prime numbers: [3,5] (for example, 7-4 = 3, 9-4 = 5).


However, it seems it oscillates between subtraction and addition. 7 - 4 = 3, but 4 + 5 = 9. I'd assume the next sequence would be 7, which is an odd prime number. And the next step requires subtraction, so it is: 9 - 7 = 2. Therefore, the next number would jump by 2 units (21 + 2). Which ends up in 23, which is the letter W

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    $\begingroup$ If you allow for such complicatedness in the answer, starting from such a small data set, then there are hundreds of similar valid answers. $\endgroup$ Mar 22, 2019 at 12:55

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