1, 1, 2, 3, 4, 5, 8, 13, 21 - which one doesn't belong to this series?

A series is a set of numbers following a pattern.

Answer: The number 4 does not belong to this series 1, 1, 2, 3, 4, 5, 8, 13, 21.

Let us understand the rule of the series.

Explanation:

The given series is a Fibonacci Sequence: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, ...

• 0 + 1 = 1
• 1 + 1 = 2
• 1 + 2 = 3
• 2 + 3 = 5 
• 3 + 5 = 8 and so on...

Rule: The next number is found by adding the two numbers before.

So, 4 is not following that rule, like 2 + 3 ≠ 4 and 3 + 4 ≠ 5 

Thus, the number 4 does not belong to this series 1, 1, 2, 3, 4, 5, 8, 13, 21.