Question 979858
Your series should actually be:
 
1, 11, 21, 1211, 1112<u><b>2</b></u>1 , ______?
 
To do these types of questions, you look at the amount of times a number appears in each term. 
 
So for the term "1", there is "one, one", which numerically written would be "11".
 
In the term "11", there are "two, ones", which numerically written would be "21".  
 
In the term "21", there is "one, two" and "one, one", which numerically written would be "1211"
 
In the term "1211", there is "one, one", "one, two" and "two, ones", which numerically written would be "111221"
 
Now, I will leave you to answer the question yourself.