This is a simple division series; each number is one-half of the previous number.
In other terms to say, the number is divided by 2 successively to get the next result.
4/2 = 2
2/2 = 1
1/2 = 1/2
(1/2)/2 = 1/4
(1/4)/2 = 1/8 and so on.
The given sequence is a combination of two series :
I. 11, 21, 32, 41, 51 and
II. 2, 3, 4, 5, 6.
Clearly, the pattern in I is + 10.
So, 32 is wrong and should be replaced by (21 + 10) i.e. 31