My Math Forum relationship between numbers

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 September 6th, 2014, 12:30 AM #1 Newbie   Joined: Sep 2014 From: I Posts: 3 Thanks: 0 relationship between numbers Hi what is the next number after "20"? 3,5,5,8,16,20,..?.. a)24 b)28 c)30 d)60 please also say the reason of your choice. thanks.
 September 6th, 2014, 05:34 PM #2 Math Team   Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 9,449 Thanks: 640 3,5,5,8,16,20,36,41 odd spots : 3,5,16,36 ; 2,11,20 = differences even spots: 5,8,20,41 : 3,12,21 = differences Justa wild attempt/guess !! Thanks from asgharshabro
 September 6th, 2014, 05:47 PM #3 Senior Member   Joined: Jan 2014 From: The backwoods of Northern Ontario Posts: 367 Thanks: 68 Here is one solution in which the next number would be 36. List the original sequence: 3,5,5,8,16,20 List the differences between consecutive numbers: 2,0,3,8,4 List the numbers in the odd positions: 2,3,4 (These are the positive integers beginning with 2) List the numbers in the even positions: 0, 8 (These are every 8th non-negative integer) So the next number in an even position will be 16. So 16 will be the difference between 20 and the next number in the original sequence. Thus the next number will be 36. Uh oh. I just posted my solution, and then saw that Denis had beat me to it! We seem to have gotten the same results, but by a slightly different method. Thanks from asgharshabro Last edited by Timios; September 6th, 2014 at 05:54 PM. Reason: Denis' post
September 7th, 2014, 05:33 AM   #4
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Quote:
 Originally Posted by Denis 3,5,5,8,16,20,36,41 odd spots : 3,5,16,36 ; 2,11,20 = differences even spots: 5,8,20,41 : 3,12,21 = differences Justa wild attempt/guess !!
But he needs the answer to be one of the options he provided. '36' isn't one.

 September 7th, 2014, 06:18 AM #5 Newbie   Joined: Sep 2014 From: I Posts: 3 Thanks: 0 Tnx Denis and Timios I think you are correct. "36" is the correct answer but it is not mentioned in choices and question isn't true. It was a question on university entry test in Iran.
September 7th, 2014, 07:10 AM   #6
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Quote:
 Originally Posted by Korisnik But he needs the answer to be one of the options he provided. '36' isn't one.
He forgot option : e) 36 !!

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