Aptitude - Odd Man Out and Series - Discussion

Discussion Forum : Odd Man Out and Series - Find Wrong No. (Q.No. 7)
Directions to Solve
Find out the wrong number in the series.

7.
1, 3, 10, 21, 64, 129, 356, 777
10
21
64
129
356
Explanation:

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

So, 356 is wrong.

Discussion:
10 comments Page 1 of 1.

Dhara said:   5 years ago
I think 64 should be the answer.

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

Aditya Kavlekar said:   5 years ago
As per my logic;.

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

Anu said:   6 years ago
Thanks for the explanation.

Ankush said:   7 years ago
Agree @Imran.

Kuldeep choudhary said:   7 years ago
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.

Sarvani said:   7 years ago
64 is a perfect square number, rest all are not.

Sweety said:   10 years ago
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.
(1)

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
(3)

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..
(1)