Results 1 to 10 of 10

Thread: Digit sum

Hybrid View

Previous Post Previous Post   Next Post Next Post
  1. #1
    Junior Member
    Join Date
    Aug 2007
    Answers
    2

    Post Digit sum

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

    1999 2001
    2 x 5


  2. #2
    Contributing Member
    Join Date
    Sep 2006
    Answers
    962

    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

    ------------------
    suresh


  3. #3
    Contributing Member
    Join Date
    Sep 2006
    Answers
    962

    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?

    ----------------
    suresh


  4. #4
    Junior Member
    Join Date
    Aug 2007
    Answers
    2

    Re: Digit sum

    Yes. You R Correct


  5. #5
    Contributing Member
    Join Date
    Sep 2006
    Answers
    962

    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

    ---------------------
    suresh


  6. #6
    Junior Member
    Join Date
    Jul 2007
    Answers
    14

    Re: Digit sum

    great thinking suresh..


  7. #7
    Junior Member
    Join Date
    Aug 2009
    Answers
    2

    Smile Re: Digit sum

    2^1999=(2^2000)/2

    5^2001=(5^2000)*5

    lhs={(2*5)^2000}*5/2

    (10^2000)*2.5
    {(10^2)^1000}*2.5

    (100^1000)*2.5
    (1 followed by 20 zeroes)*2.5

    25 followed by 19 zeroes

    hence, sum of digits =2+5=7

    just getting the advantage of particular set of 2 & 5


  8. #8

    Re: Digit sum

    Quote Originally Posted by rahul14904 View Post
    2^1999=(2^2000)/2

    5^2001=(5^2000)*5

    lhs={(2*5)^2000}*5/2

    (10^2000)*2.5
    {(10^2)^1000}*2.5

    (100^1000)*2.5
    (1 followed by 20 zeroes)*2.5

    25 followed by 19 zeroes

    hence, sum of digits =2+5=7

    just getting the advantage of particular set of 2 & 5
    Morning
    I'm trying to do same problem guys
    How did you get that:
    (100^1000) = 1 followed by 20 zeros
    Because what I got from 10^n
    Letting n=1, 10
    n=2, 100
    .
    .
    .
    n=10, 1 followed by 10 zeros


  9. #9
    Junior Member
    Join Date
    May 2007
    Answers
    19

    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.


  10. #10
    Junior Member
    Join Date
    Aug 2009
    Answers
    2

    Re: Digit sum

    2^1999=(2^2000)/2

    5^2001=(5^2000)*5

    lhs={(2*5)^2000}*5/2=(10^2)^1000*2.5=(100^1000)*2.5=(1 followed by 20 zeroes)*2.5=25 followed by 19 zeroes=>this gives sum of digits=2+5=7


Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
About us
Applying for a job can be a stressful and frustrating experience, especially for someone who has never done it before. Considering that you are competing for the position with a at least a dozen other applicants, it is imperative that you thoroughly prepare for the job interview, in order to stand a good chance of getting hired. That's where GeekInterview can help.
Interact