A Tougher Series
Find the missing number in the series:
3 - 11 - 20 - 27 - 29 - 23 - ?
3 - 11 - 20 - 27 - 29 - 23 - ?
posted by Zaux at
10:46 AM
Tuesday, February 23, 2010
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11 Comments:
6
The solution to the "not so tough series" is 6 as was so precisely stated by "rarely puzzled" Chris
:-)
6 is correct.
The sequence is 3,11,20,27,29,23
The first differences are 8,9,7,2,-6
The second differences are 1,-2,-5,-8
The third differences are -3,-3,-3
finally, a pattern. So:
the next 2nd difference is -8-3 = -11
The next 1st difference is -6-11 = -17
The next number in the sequence is 23-17 = 6
nice explanation Ross
Hi Ross. I think it's easier to use Cam's trick of using x to
represent the number. You'd have got for the last line:
-3,-3,-3,x-9. So x-9 = -3 => x = 6
You're right, that is easier. Thanks.
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Ross. My pleasure. I guess you didn't see the Not the World Series, but ... problem. Take a look, it's quite a treat.
Soz about the deletes, but the link keeps on formatting improperly.
It's practically essential to do that trick if you've got more than one blank to fill.
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