number system

This topic has expert replies
Source: — Problem Solving |

Junior | Next Rank: 30 Posts
Posts: 15
Joined: Sun Jul 20, 2008 6:37 am

by Saule » Wed Jul 23, 2008 3:03 am
Hi,
The remainder is 5, hence the answer is E, none of these.

Junior | Next Rank: 30 Posts
Posts: 15
Joined: Sun Jul 20, 2008 6:37 am

by Saule » Wed Jul 23, 2008 3:11 am
Let me explain my logic.

9^1 + 9^2 + 9^3 + ...... + 9^9 = 9 (9^0 + 9^1 + 9^2 +.....+ 9^8 )
Divide this by 6.
Eliminate 3 from both numerator and denominator, and you'll be left with 2 in the denominator.
When you divide a number by 2 you can only have 0 in the remainder if the number is even or 5 if it's odd.
So is 3 (9^0 + 9^1 + 9^2 +.....+ 9^8 ) even or odd?
It's odd, because 9 to any power is always odd and you have it nine times in the brackets, so odd * 9 = odd, and then 3 * odd = odd.
So the remainder is 5.
Last edited by Saule on Wed Jul 23, 2008 3:18 am, edited 2 times in total.

Legendary Member
Posts: 829
Joined: Mon Jul 07, 2008 10:09 pm
Location: INDIA
Thanked: 84 times
Followed by:3 members

by sudhir3127 » Wed Jul 23, 2008 3:13 am
My answer is 3.

let me know its right?

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 2623
Joined: Mon Jun 02, 2008 3:17 am
Location: Montreal
Thanked: 1090 times
Followed by:355 members
GMAT Score:780

Re: number system

by Ian Stewart » Wed Jul 23, 2008 3:23 am
nikhilagrawal wrote:What is the reminder when 9^1 + 9^2 + 9^3 + ...... + 9^9 is divided by 6?
We're adding nine odd numbers here- the sum will be odd. The sum will also be a multiple of 3, since each term in the sum is divisible by 3. So the sum is an odd multiple of 3, and the remainder will be 3 when you divide by 6.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com

Junior | Next Rank: 30 Posts
Posts: 15
Joined: Sun Jul 20, 2008 6:37 am

Re: number system

by Saule » Wed Jul 23, 2008 3:34 am
Ian Stewart wrote:
nikhilagrawal wrote:What is the reminder when 9^1 + 9^2 + 9^3 + ...... + 9^9 is divided by 6?
So the sum is an odd multiple of 3, and the remainder will be 3 when you divide by 6.
I have to disagree that the remainder is 3 when you divide an odd multiple of 3 by 6.
Divide for example 9 by 6, you get 1.5

Legendary Member
Posts: 661
Joined: Tue Jul 08, 2008 12:58 pm
Location: France
Thanked: 48 times

by pepeprepa » Wed Jul 23, 2008 3:41 am
Just one remark about Saule.
I think I see where your error is.

You say "Eliminate 3 from both numerator and denominator, and you'll be left with 2 in the denominator."
I think you cannot do that when you want to find a remainder.
Counter-exemple:
15 divided by 6 gives you a remainder of 3
15/6=(3*5)/(3*2)=5/2
5 divide by 2 gives you a remainder of 1

Am I right about this fact?

Senior | Next Rank: 100 Posts
Posts: 81
Joined: Thu Jun 12, 2008 1:57 pm
Thanked: 1 times

by nikhilagrawal » Wed Jul 23, 2008 3:43 am
Sal

Legendary Member
Posts: 661
Joined: Tue Jul 08, 2008 12:58 pm
Location: France
Thanked: 48 times

by pepeprepa » Wed Jul 23, 2008 3:44 am
"I have to disagree that the remainder is 3 when you divide an odd multiple of 3 by 6"
That's wrong.

9 divided by 6 gives you a remainder of 3
81 (which is 3*27) divided by 6 gives you a remainder of 3

Junior | Next Rank: 30 Posts
Posts: 15
Joined: Sun Jul 20, 2008 6:37 am

by Saule » Wed Jul 23, 2008 3:53 am
pepeprepa wrote:Just one remark about Saule.
I think I see where your error is.

You say "Eliminate 3 from both numerator and denominator, and you'll be left with 2 in the denominator."
I think you cannot do that when you want to find a remainder.
Counter-exemple:
15 divided by 6 gives you a remainder of 3
15/6=(3*5)/(3*2)=5/2
5 divide by 2 gives you a remainder of 1

Am I right about this fact?
OK, the remainder should be 1, not 5. When an odd number is divided by 2, the remainder's always 1, my mistake. But the answer is still None of these, since 1 isn't an option either.

9^1 + 9^2 + 9^3 + ...... + 9^9 = 3x3 (9^0 + 9^1 + 9^2 + ...... + 9^8 )
this is our numerator.

the denominator is 6 = 3x2

cancel 3 in the numerator and denominator, and you're left with
3 (9^0 + 9^1 + 9^2 + ...... + 9^8 ) divided by 2.

Since the numerator is odd, the remainder is 1.

Legendary Member
Posts: 661
Joined: Tue Jul 08, 2008 12:58 pm
Location: France
Thanked: 48 times

by pepeprepa » Wed Jul 23, 2008 3:59 am
But Saule what do you think about my counter-example?
I mean when you reduce numerator and denominator by the same factor, the remainder is not the same for both equations.

Junior | Next Rank: 30 Posts
Posts: 15
Joined: Sun Jul 20, 2008 6:37 am

by Saule » Wed Jul 23, 2008 4:46 am
You're right! I just realized when you reduce a fraction by some number, the reminder is also reduced by the same number.
So in end we need to multiply 1 by 3 and get 3.

Thank you, pepeprepa!