Directions (61-65) : Find the wrong number in following number series.

4 , 9 , 18 , 35 , 69 , 139, 277

The correct option is 18.

To find the wrong number in the given series, let us analyze the pattern connecting consecutive terms:

First term: 4

Second term:

$4\times 2+1=8+1=9$

Following this pattern, each subsequent term should be obtained by multiplying the previous term by 2 and alternately adding 1 or subtracting 1:

Third term should be:

$9\times 2-1=18-1=17$

However, the given series has 18 in place of 17.

Let us verify the remaining terms assuming the correct term is 17:

Fourth term:

$17\times 2+1=34+1=35$

Fifth term:

$35\times 2-1=70-1=69$

Sixth term:

$69\times 2+1=138+1=139$

Seventh term:

$139\times 2-1=278-1=277$

All subsequent terms match the given series when 18 is replaced by 17. Therefore, 18 is the wrong number in the series.