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Digit sum

This is a discussion on Digit sum within the Challenging puzzles forums, part of the Brain Gym category; What is the sum of the digits of the decimal form of the product below? 1999 2001 2 x 5...


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Old 08-22-2007
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Post Digit sum

What is the sum of the digits of the decimal form of the product below?

1999 2001
2 x 5
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Old 08-23-2007
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Re: Digit sum

Hi,

Your problem is not clear. what you want actually? what is the meaning of the following...

1999 2001
2 x 5

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suresh
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Old 08-23-2007
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Re: Digit sum

I think you need the solution for the following....

(2^1999 * 5^2001) Sum of the results of this product...

Am i correct?

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suresh
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Old 08-23-2007
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Re: Digit sum

Yes. You R Correct
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Old 08-24-2007
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Re: Digit sum

The answer for your problem is 7.

Solution:

Your problem is (2^n * 5^(n+2))

Let's take n=1
result is 2*125 = 250 = 7

Let's take n=2
result is 4*625 = 2500 = 7

Let's take n=3
result is 8*3125 = 25000 = 7

Let's take n=4
result is 16*15625 = 250000 = 7
.
.
.
So finally we got the result like 250...... and the sum is 7

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suresh
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Old 08-24-2007
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Re: Digit sum

That's a Great Logic!
Thnks.
2^n*5^(n+2)
= 2^n*5^n*5^2
=(2*5)^n*25
=10^n*25
Sum is always 7.
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Old 08-24-2007
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Re: Digit sum

great thinking suresh..
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