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DS.. properties of numbers

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DS.. properties of numbers [#permalink] New post 09 Mar 2006, 12:42
00:00
A
B
C
D
E

Difficulty:

(N/A)

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oMY.. I thought I knew how to solve this problem. But in the end I was wrong.

Can some one help me with this?:

What is the value of 1/ k + 1/ r?

k+r = 20

kr= 64.

I thought i was correct when i just did
1) k + r =20 ==> 1/k +1/r =1/20.
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 [#permalink] New post 09 Mar 2006, 12:55
E.

(1) k+r = 20
say k=10 and r=10, 1/k + 1/r = 1/5
say k=15 and r=5, 1/k + 1/r = 4/15
INSUFFICIENT

(2) kr = 64
same as above, k and r could be any two numers that are factors of 64
INSUFFICIENT

combined, 4 and 16 satisfy this, but so do -4 and -16
1/4 + 1/16 = 5/16
-1/4 + -1/16 = -5/16
INSUFFICIENT

Ans. E
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 [#permalink] New post 09 Mar 2006, 13:18
What's the OA?

Is it C?
1/k + 1/r = (r+k)/kr

we know r+k and kr from both statements.
So C
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 [#permalink] New post 09 Mar 2006, 13:22
Nayan wrote:
What's the OA?

Is it C?
1/k + 1/r = (r+k)/kr

we know r+k and kr from both statements.
So C


Ah very good. I just re-read my statement. should be C. there's no way it could be -4 and -16 because k+r = 20.

What is the official answer Angela780?

Last edited by kook44 on 09 Mar 2006, 13:36, edited 1 time in total.
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 [#permalink] New post 09 Mar 2006, 13:51
Is the Ans C.

1/k + 1/r can be written as (k+r)/kr
Stmt 1 gives only the value k+r : Not sufficient
Stmt 2 Gives only the value of kr : Not sufficient

Both stmts. together we can find the value og 1/k + 1/r = 20/64
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 [#permalink] New post 09 Mar 2006, 17:03
OA is C

Thank you to everyone that helped with this problem.

But i still struggle to see why it is (r+k)/kr
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 [#permalink] New post 09 Mar 2006, 19:19
Angela780 wrote:
OA is C

Thank you to everyone that helped with this problem.

But i still struggle to see why it is (r+k)/kr


Hey Angela,

It's nice to see your posts :)

It is this way...

1/k+1/r, for this addition we need to have denominator of both terms same, i.e. LCM, & LCM for this is "kr"

=> (r/kr)+(k/kr) -------------- (multiply & divide a term by same number so that we get denominator "kr")

Now we can do addition,
(r/kr)+(k/kr) = (r+k)/kr

Is it ok?
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 [#permalink] New post 09 Mar 2006, 19:28
(1/k) + (1/r)


In order to get a common denominator rk

multiply first term by r/r and second term by k/k

then we will have

(r/rk) + (k/kr) which is same as (r/kr) + (k/kr)

=> (since we have a common denominator we can add all together now)

(r+k)/kr

example would be the addition of 1/2 + 1/3

can be re-written as 2/2*3 + 3/3*2

which is 2/6 + 3/6 => 5/6

Hope it helps
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 [#permalink] New post 10 Mar 2006, 02:41
oooo yes yes... I get it now.. :-D ,

Thank you all very much.... the dust is lifting from my brain...hahahah
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  [#permalink] 10 Mar 2006, 02:41
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