Hi anurag0928,
Any time you're not completely sure of how a particular math concept 'works', it can often be helpful to come up with a few examples (to prove that what you think about a concept is actually correct).
Here, we're dealing with |Y| = -Y.
Could Y = 0?
|0| = 0
-(0) = 0
The results (re: 0 and 0) are equal, so Y CAN equal 0 here.
Could Y = 1?
|1| = +1
-(1) = -1
The results (re: +1 and -1) are NOT equal, so Y CANNOT equal 1. Based on this example, you can probably see that the same general result will occur whenever Y is a positive number (re: the resulting calculations will NOT be equal)... meaning that there are NO positive values for Y that fit this equation.
Could Y = -1?
|-1| = +1
-(-1) = +1
The results (re: +1 and +1) are equal, so Y CAN equal -1 here. Based on this example, you can probably see that the same general result will occur whenever Y is a positive number (re: the resulting calculations WILL be equal)... meaning that ALL NEGATIVE values of Y are solutions to this equation.
Thus, we know that Y could be 0 or any negative number, but NOT any positive number. That's Y ≤ 0.
GMAT assassins aren't born, they're made,
Rich