Question

Find the next number in the series and explain its logic.

5,9,21,37,81,?

Answer 1

Ans: 153 
Bcoz the difference between successive no.s is 
2* previous number alternate +(or)- (2n-1)th number where n=1,2,3,......... 
Ex: 5*2 - 1=10-1=9 (2nd number) 
9*2 + 3=18+3=21(3rd number) 
21*2 - 5=42-5=37(4th number) 
37*2+ 7=74+7=81(5th number) 
hence the 6th number would be 81*2 - 9= 162-9=153

Answer 2

It continues like this.....

$5 \times 2 - 1 = 9$
$9 \times 2 + 3 = 21$
$21 \times 2 - 5 = 37$
$37 \times 2 + 7 = 81$
$81 \times 2 - 9 = 153$

This is the logic behind this question.