
Elementary Math Fractions, Percentages, Word Problems, Equations, Inequations, Factorization, Expansion 
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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: 12,396 Thanks: 829 
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 !! 
September 6th, 2014, 05:47 PM  #3 
Senior Member Joined: Jan 2014 From: The backwoods of Northern Ontario Posts: 371 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 nonnegative 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. Last edited by Timios; September 6th, 2014 at 05:54 PM. Reason: Denis' post 
September 7th, 2014, 05:33 AM  #4 
Newbie Joined: Sep 2014 From: . Posts: 15 Thanks: 2  
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 
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 12,396 Thanks: 829  

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