How in the world?

notgoodinmath · Posted: Tue Jul 08, 2008 3:48 pm Post subject: How in the world?

Can someone please tell me what is the best strategy to use when answering these questions:

If x-y=8, which of the following must be true?
I. Both x and y are positive
II. If x is positive, y must be positive
III. If x is negative, y must be negative

I only
II only
III only
I and II
II and III

Thanks!

duongthang · Posted: Tue Jul 08, 2008 7:25 pm Post subject:

pick the specific numbers . OA is 3 only

preetha_85 · Posted: Tue Jul 08, 2008 8:30 pm Post subject: Solution

you have got to pick nos. and check

case 1 : both x&y are positive
assume x=6;y=-2.
then x-y=6-(-2)=8

Hence 1 is not the answer.

Case 2 : If x is positive ,y must be positive
Not necessary.. from the example above

Case 3: If x is negative y must be negative
Assume x=-7;then y has to be negative for the answer to be positive.
since only then will the sins cancel out for the no. to be positive.
So in this case if y=-15
x-y= -7-(-15) = 8

Hence option 3 is right

VP_Jim · Posted: Tue Jul 08, 2008 8:36 pm Post subject:

I agree with the above - the best way to do this (and most other problems involving variables) is to pick numbers.

This question tells us that x-y=8. So, we can pick numbers such as:

x=10, y =2
x = 4, y = -4
x = -2, y=-10

etc. etc.

Then, plug these numbers into the answer choices and eliminate ones that come out "false". You may have to do multiple iterations of this before you arrive at the correct answer.

amitdgr · Posted: Wed Jul 09, 2008 5:14 am Post subject: Re: How in the world?

Take case I : Both x and y are positive ----- Need not be, Y can be negative eg. x=4 and y=-4 => x-y = 4+4 = 8

Take Case II : If x is positive, y must be positive ----- Disproved above

Case III - If x is negative, y must be negative ----- If x is negative and y is positive, then -x-y will give a negative result. This shows that if x is negative, y must be negative so that the eqn becomes -x+y which gives us a possibility of having positive result.

Right answer is C. III only

HTH

Amit

notgoodinmath · Posted: Wed Jul 09, 2008 11:48 am Post subject:

Thanks Guys! time consuming for sure

Stuart Kovinsky · Posted: Wed Jul 09, 2008 1:36 pm Post subject:

albertrahul · Posted: Thu Jul 10, 2008 2:34 am Post subject: Thanks Stuart!

This is such an important strategy for saving time.
Thanks Stuart!! I've been spending awful lot of time in such problems.