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Source: — Data Sufficiency |

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Re: Help: Powerprep question

by lunarpower » Sun May 04, 2008 11:32 am
caramel3536 wrote:Is rst =1?
(1) rs = 1
(2) st = 1

ans E
(1)
insufficient, because you don't know anything about t

(2)
insufficient, because you don't know anything about r

(together)
if you multiply the two statements together, you get (r)(s^2)(t) = 1, but this isn't the same as rst = 1.
so, try to find contradictory examples:
if r = 1/2, s = 2, t = 1/2, then rst = 1/2 (so doesn't equal 1)
if r = s = t = 1, then rst = 1
insufficient

answer = e
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by caramel3536 » Sun May 04, 2008 2:07 pm
Thanks a lot
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by anniev2 » Wed Mar 11, 2009 8:27 pm
Why can't the equations provided in the statement be plugged into the given:

Is rst=1

(Statement 1) rs=1

Substituting values in equation:
1 x t =1
1t=1
t=1
- if we know that rs = 1 and t = 1 then we know that rst = 1

...
same can be done for the second statement. Why is this unacceptable?
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by Ian Stewart » Thu Mar 12, 2009 3:47 am
anniev2 wrote:Why can't the equations provided in the statement be plugged into the given:

Is rst=1

(Statement 1) rs=1

Substituting values in equation:
1 x t =1
1t=1
t=1
- if we know that rs = 1 and t = 1 then we know that rst = 1

...
same can be done for the second statement. Why is this unacceptable?
"Is rst = 1?" is the question; it's not a fact that you can use. If you do assume "rst = 1" is true here, it should not be a surprise to discover that rst is equal to 1 -- that's what's called 'begging the question' in logic (assuming the answer to a question is yes, then proving the answer is yes).

When you plug rs = 1 into the question to find "t = 1", all you are doing is rephrasing the question. That is, if we use Statement 1 here, our question becomes: "is t equal to 1?" Since we don't know if that's true, Statement 1 is not sufficient.
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by HPengineer » Thu Dec 23, 2010 6:52 pm
I used the following method that all three numbers are 1 and that based on the statements they just have to be the same sign.

From stm 1 R x S = 1

could be -R x -S = 1 or R x S = 1


From stm2

could be S x T = 1 and -S x -T = 1

Stm 1 & 2 together
you can have RST = 1 or -R-S-T = -1

Did i get lucky here or is this a valid approach?
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