# 1, 1, 2, 3, 4, 5, 8, 13, 21 – Which one doesn’t belong to this series?

• This is a Fibonacci number

we know that

1

1 + 1 =2

1 + 2 = 3

2 + 3 = 5

3 + 5 = 8

5 + 8 = 13

13 + 8 = 21

Therefore the number doesn't belong to this series is 4....

• 21-13=8

13-8=5

8-5=3

5-3=2

3-2=1

2-1=1

now you decide which number

does not belong to this set of numbers.

• This is a fibonacci series every number is the sum of the previous two numbers. the 5 does not belong.

• If you take out 4 then every number is the sum of the two before it.

• Everyone thinks it is funny to give you the wrong answer 4.  Don't let them make fun of you.  The correct answer is 5.  The people who answered think it is far more important that they look smart than it is that you try to figure it out and actually learn.

• number 4 , it can only be used once .

• 4 doesn't belong to the series: 1, 1, 2, 3, 4, 5, 8, 13, 21, ..

• 4.

1+1 is 2

1+2 is 3

2+3 is 5

5+3 is 8

And so on.

• If you take out the 4, you have the start of the Fibonacci Sequence, which is defined as each term is defined as the sum of the previous two terms (Except the first 2 terms which are each 1)

So the 4 doesn't belong.

• First, that's a sequence, not a series, since there are no addition signs.

Second, any answer can be justified, for example 1 doesn't belong because it's the only number that's repeated, or 21 doesn't belong because the rest of the numbers aren't divisible by 7, etc.

However, I do notice that this sequence looks exactly like the Fibonacci sequence, except that 4 is not part of the Fibonacci sequence. In the Fibonacci sequence, the first two numbers are 1, and every other number is the sum of the two previous numbers. So in that case, the answer is 4.