Find Next Number

Puzzle:

Find the next number in the series.

• 1211
• 122111
• 312211
• 12221131
• ?

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Solution:

Answer is 1113213211.

Just reverse numbers 2nd and 4th (alternative) and look at that series.

• 1211
• 111221
  
• 312211
• 13112221
  

So after starting the sequence with 1211, each term in the sequence consists of groups of two numbers based on the previous term - the first being the quantity and the second specifying which digit. For example, first term has "one 1", "one 2" and "two 1s". So the second numbers becomes 111221. Now the 5th number is 1113213111. Since we reverse only even terms, the answer is 1113213211.