Aptitude - Odd Man Out and Series - Discussion

Find out the wrong number in the series.

10. 

3, 7, 15, 39, 63, 127, 255, 511

[A]. 7
[B]. 15
[C]. 39
[D]. 63
[E]. 127

Answer: Option C

Explanation:

Go on multiplying 2 and adding 1 to get the next number.

So, 39 is wrong.


Ashik Mohammed Nazar said: (Jun 7, 2011)  
3,7,15,39,63,127,255,511
If u closely look at this pattern,and the differences between the consecutive numbers,
It is 4 8 24 24 64 128 256
By changing 39 to 31, the differences become
4 8 16 32 64 128 256 which is
2^2 2^3 2^3 2^4 and so on ...

Shruti said: (Aug 22, 2012)  
It can also be solved like such---
2^2-1,2^3-1,2^4-1,2^5-1 and so on....

Grace said: (Jan 28, 2014)  
3+4 = 7.
7+8 = 15.
15+16 = 31.
31+32 = 63.
63+64 = 127.
127+128 = 255.
255+256 = 511.
.
.

Siva said: (Dec 20, 2015)  
Please clear explain.

Mehtab said: (Apr 7, 2016)  
3*2 + 1 = 7
7*2 + 1 = 15
15*2 + 1 = 31
31*2 + 1 = 63
63*2 + 1 = 127
127*2 + 1 = 255
255*2 + 1 = 511

31 is given instead of 31.

39 is the wrong answer.

Poooja said: (Jun 30, 2016)  
Thank you for your clear answer.

R@M said: (Jul 18, 2016)  
The another logic is 2^2 - 1, 2^3 - 1, 2^4 - 1, .....

Nani said: (May 1, 2018)  
All are divisible by 3. But 7 can't be divisible. So, why can't that be the answer?

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