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Sequence?

Posted by Karl Sharman on February 1, 2011 – 3:02 pm

Next numbers in the sequence – 15, 21, 25, 27, 33, 35, 39, ? ,? ,?

I don’t have an answer for this… yet. So, try and convince me with a plausible answer…

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  1. 1. Chris Said:

    wolframalpha suggests:
    15,21,25,27,33,35,39,
    45,161/3,723/11,1065/13,103,2205/17,3093/19,203,…
    LOL

  2. 2. Karl Sharman Said:

    I checked wolfram as well. Not convinced!

  3. 3. Bekki Said:

    Stop cheating and using wolfram.

    alternating numbers: 15,25,33,39–> they go up by 10,8,6 so next would be up by 4 –> 43
    21,27,35–> they go up by 6,8,so next would be up by 10.–> 45

    so total would be
    15 25 33 39 43 45 …
    21 27 35 45 57 …

    15,21,25,27,33,35,39,45,43,57,45…

  4. 4. Chris Said:

    Hi Bekki, I wasn’t really cheating. I knew that it wasn’t going to give a sensible answer before I looked.

    Nice one though, you just beat me to a plausible solution.

  5. 5. cazayoux Said:

    I guess the next three are 45, 45, 49.
    Then continuing with …
    51, 55, 57, 63, 65, 75, 77, 81, 85, …

  6. 6. cazayoux Said:

    seems like we are listing the product of two odd numbers, sorted by the product.

    3*5, 3*7, 5*5, 3*9, 3*11, 5*7, 3*13 …
    3*15, 5*9, 7*7 would be next.

  7. 7. Chris Said:

    Hi cazayoux. Nice one, I hadn’t thought of that – despite writing the sequence as prime factorisations.

    But I’d have gone for odd primes rather than odd numbers. But, either way, I’d have expected the sequence to have started at 9, not 15.

    So I’d go for 3*17 (51), 5*11 (55), 3*19 (57), 5*13 (65),…

  8. 8. nuhbye Said:

    I think Next numbers in the sequence – 15, 21, 25, 27, 33, 35, 39 is governed as…

    take every odd number from the starting i.e. 15, 25, 33, 39. The difference b/w two numbers (10, 8, 6) is increasing by 2 so the next number is this sequence is 43, 45, 47.

    Similarly in the even placed positions 21, 27, 35 the difference (6, 8) is increasing by 2 so the next two numbers are 45, 57

    So the final sequence is 15, 21, 25, 27, 33, 35, 39, 45, 43, 57.

  9. 9. John Said:

    I guess the series goes like this:
    after 15, we have 1 number (21), then, 15+10=25,
    after 25, we have 2 numbers (27,33), then 25+10=35,
    hence after 35, we have 3 (39,x,y), then 35+10=45.

    The numbers in between 15,25,35,… are obtained by adding 6, ie, 21+6=27, 27+6=33, 33+6=39, hence the next two numbers are:
    39+6=45, and 45+6=51

    therefore the series is:
    15, 21, 25, 27, 33, 35, 39, 45, 51, 45

  10. 10. SEN23 Said:

    THE CORRECT ANSWER IS 45,49,51 THE SEQUENCE IS UP 6 UP 4 UP 2 IDIOTS

  11. 11. adnan Said:

    next num 45,49,51

  12. 12. Karl Sharman Said:

    I think Sen23 has it… I’m an idiot for not spotting it earlier:

    15 to 21 – up 6
    21 to 25 – up 4 – I see where this is going….
    25 to 27 – up 2 – wow this look promising…
    27 to 33 – up 6 – on fire! I’m an idiot for not seeing this…
    33 to 35 – up 4 – no wait, hang on…

    So close…..

    PS i wrote this on my brambleberry phone – so think how long it took me to type this out, what with my fat fingers and limited intelligence….
    Holiday is great, Cairo is warm, almost hot you could say!

  13. 13. Chris Said:

    Hi SEN23. Only the first three terms do that.

    Ask your nurse to sponge the dribble off your keyboard. You’ll then be able to find the caps lock key.

  14. 14. Jamie Said:

    #1+6, #2+4, #3+2
    so it would be
    15, 21, 25, 27, 33, 35, 39, 45, 49, 51….

  15. 15. Jamie Said:

    Sorry I see I was wrong my brain skipped right over the error

  16. 16. Antn'y Said:

    45, 49, 51

    I read this as the sequence of numbers which is neither even nor prime (and not less than 10). Although this is a nice looking open ended problem, there could be lots of things.

  17. 17. cosnard Jerome Said:

    15 21 25 27 33 35 39 45 49 51 55 57 63 65 69 75 77 81 85…

  18. 18. Neha Sudhir Said:

    They are all odd composite nos. So the next three nos. Are 45, 49, 51.( Odd composite numbers start from 9….since 3,5, 7 are prime nos.)

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