What is the next number in the sequence?
1
11
21
1211
111221
312211
13112221
Answer
The next number in the sequence is 1113213211.
Each number describes the previous number. Starting with 1, the second number describes it as 11 (one 1). Then the third number describes 11 as 21 (two 1’s). Then the fourth number describes 21 as 1211 (one 2, one 1).
To describe 13112221, it has to be 1113213211 (one 1, one 3, two 1, three 2, one 1).
In mathematics, this is called the look-and-say sequence.