Paper test code 55, Section 3, Question 16

Question

This question is taken from the GMAT paper test code 55 and it makes no sense to me.

16.
For any numbers a and b, axb = a+b-ab.
If axb=0, which of the following CANNOT de a value of b?
a)2
b)1
c)0
d)-1
e)-3/2

Oski · Answer

Actually it makes perfect sense.

axb=0 gives you a+b-ab=0 that you can write a+b=ab

This equation is impossible for b=1, it would need to be true to find a number a, such as a+1=a

==> Answer is (B)

Edit: from a+b=ab you can write  a=\frac{b}{b-1}

You can find a solution to this equation for every b different than 1.

DFG5150 · Answer

Thanks Oski, your answer matchs the OA, but I still don´t get something:

if axb=0, that means that:
1. a+b=0 (not a+b-ab, since ab=axb)
2. Either a or b is 0

So, the onlny possible answer here is that a=b=0.

¿What am I getting wrong?

Oski · Answer

The question defines a new operation "x" such as: axb = a+b-ab

You have to take it for granted, the x operation here is not the multiplication

So when they tell you axb=0, just use their definition and you get a+b-ab = 0

kapilnegi · Answer

Friend, axb is a decoy. here 'x' can be anything... 'x'it doesn't imply multiplication. I hope this clears the haze.

DFG5150 · Answer

Damn!!!

Sorry for the stupid question!! Is that suposed to be obvious to everyone?....

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