## Missing number in the series

This is a discussion on Missing number in the series within the Brainteasers forums, part of the Brain Quiz category; What is the missing number in the series? 4,9,20,43,90??...

1. Contributing Member Array
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## Missing number in the series

What is the missing number in the series?
4,9,20,43,90??

2. Expert Member Array
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425

## Re: Missing number in the series

Ans: 185.

diff between 4 and 9 is 5 (4+1)
diff between 9 and 20 is 11 (9+2)
diff between 20 and 43 is 23 (20+3)
diff between 43 and 90 is 47(43+4)

-- James

3. Contributing Member Array
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## Re: Missing number in the series

James your approach is prefect and the solution for this problem is 185 as you found out.

4. Junior Member Array
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## Re: Missing number in the series

Originally Posted by scott
What is the missing number in the series?
4,9,20,43,90??
Another approach

(4*2) + 1 = 9
(9*2) + 2 = 20
(20*2) + 3 = 43
(43*2) + 4 = 90
(90*2) + 5 = 185

5. Expert Member Array
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303

## Re: Missing number in the series

yep

current No - (90 + 5)

Prev No - 90

so 90 + 95 = 185

6. Junior Member Array
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## Missing number in the series

Originally Posted by scott
What is the missing number in the series?
4,9,20,43,90??
the next no of the series is 143

Last edited by sadaf; 05-14-2009 at 07:35 AM.

7. Junior Member Array
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## Missing number in the series

Originally Posted by scott
What is the missing number in the series?
4,9,20,43,90??
the last no of the series is 143

8. Junior Member Array
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1

## Re: Missing number in the series

4 9 20 43 90 ?

the difference

4 9 20 43 90 ?

5 11 23 47 ?

6 12 24 48

so the 3rd series is 6 12 24 (48) it doubles the digit.. adding 48 in 47 so the figure is 95 now we got main difference series.....adding 95 in main series 90 then ans is 185.

9. Expert Member Array
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## Re: Missing number in the series

Hi,

The missing number is 185.

Here is the explanation

4 9 20 43 90

9-4=5
20-9=11
43-20=23
90-43=47

(5*2)+1=11
(11*2)+1=23
(23*2)+1=47

so, in the same way (47*2)+1=95

So, x-95=90.

Therefore x= 185.

Thanks

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## Re: Missing number in the series

Originally Posted by scott
What is the missing number in the series?
4,9,20,43,90??

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## Re: Missing number in the series

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## Re: Missing number in the series

13. Junior Member Array
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## Re: Missing number in the series

How can I draw up a formula that number series?

4,9,20,43,90??
Thank you!

14. Junior Member Array
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## Re: Missing number in the series

Hi Isabele,

The formula is defined as
The difference between 9-4= 4+1 is 5
20-9=(5+5)+1 is 11
43-20=(11+11)+1 is 23
90-43=(23+23)+1 is 47
185-90=(47+47)+1 is 95

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