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September 6th, 2014, 12:30 AM   #1
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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.
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September 6th, 2014, 05:34 PM   #2
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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 !!
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September 6th, 2014, 05:47 PM   #3
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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.
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Last edited by Timios; September 6th, 2014 at 05:54 PM. Reason: Denis' post
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September 7th, 2014, 05:33 AM   #4
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Quote:
Originally Posted by Denis View Post
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.
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September 7th, 2014, 06:18 AM   #5
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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.
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September 7th, 2014, 07:10 AM   #6
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Cool

Quote:
Originally Posted by Korisnik View Post
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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