What is the next number in this sequence: 1, 11, 21, 1211, 111221, 312211, 13112221?
1113213211 is the next number in the sequence! This is called the “Look and say sequence”:
The explanation: each successive number is formed by looking at the preceding number and reading out the digits, grouping like digits together and saying the number of identical digits followed by the digit.
1 is read as “one one”, and so the second number is 11.
11 is read as “two ones”, so the third number is 21.
21 is read as “one two, one one”, so the fourth number is 1211.
1211 is read as “one one, one two, two ones”, so the fifth number is 111221.