Aptitude - Odd Man Out and Series - Discussion

Find out the wrong number in the series.

7. 

1, 3, 10, 21, 64, 129, 356, 777

[A]. 10
[B]. 21
[C]. 64
[D]. 129
[E]. 356

Answer: Option E

Explanation:

A x 2 + 1, B x 3 + 1, C x 2 + 1, D x 3 + 1 and so on.

So, 356 is wrong.


Nagendramurthy said: (Jun 20, 2011)  
Alternate method:

1=1
3=3
10=1+0=1
21=2+1=3
64=6+4=10=1+0=1
129=1+2+9=12=1+2=3
356=3+5+6=14=1+4=5( wrong one.. it should be 1)
777=7+7+7=21=2+1=3

Alternative 1 and 3 series.. this is method is correct..

Imran said: (Sep 21, 2011)  
a-1 and b=3 take first two number as a and b

1*2+1=3
3*3+1=10
10*2+1=21
21*3+1=64
64*2+1=129
129*3+1=388 but in the queston in 356 so 356 is worng
388*2+1=777

Is it clear 356 is wrong

Sweety said: (Jan 24, 2014)  
1, 3, 10, 21, 64, 129, 356, 777

3-1=2.
10-3=7.
21-10=11.
64-21=43.
129-64=65.
356-129=127.
777-356=421.

Answer should be 129. Because after subtracting in all the case give prime number except in 129-64 case.

Sarvani said: (Aug 5, 2016)  
64 is a perfect square number, rest all are not.

Kuldeep Choudhary said: (Nov 5, 2016)  
One more alternative method:-.

Separate out even & odd place numbers as 3, 21, 129, 777 & 1, 10, 64, 356.

Then find the difference between each other like 18, 108, 548 & 9, 54, 292.

Hereafter you will find the division of numbers are constant (i.e. Dividend=6), only 292 differs, then back this & 356 is the answer.

Ankush said: (May 27, 2017)  
Agree @Imran.

Anu said: (Aug 2, 2017)  
Thanks for the explanation.

Pradyumna said: (Sep 1, 2017)  
Thanks for the answers.

Aditya Kavlekar said: (Oct 28, 2018)  
As per my logic;.

Only 10 is multiple of 5 and rest are not. So odd one is 10.

Dhara said: (May 31, 2019)  
I think 64 should be the answer.

We are adding prime no 1 3 7 11 13 so 64 is wrong.

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