Q. Find the odd number in the series: 
2, 13, 65, 271, 817, 1639, 1645

Explanation by: Santosh Mishra

The given series is in the below pattern,
∴ First term = 2
Second term = ( First term × 6 ) + 1 = 13
Third term = ( Second term × 5 ) + 2 = 65 , 67 should be sixth term because ( 13 × 5 ) + 2 = 67 .
So 65 is wrong. we get 67 as Third term.
Similarly we can find the all terms
Fourth term = ( Third term × 4 ) + 3 = 271
Fifth term = ( Fourth term × 3 ) + 2 = 817
Sixth term = 1639,
Seventh term = 1645.