Preparation 
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Absolute Value and Opposites
The absolute value of a number n is denoted by n.
It is defined as the distance between 0 (zero) and n (the number) on the number line.
Importantly, the absolute value of any nonzero number is always positive.
For example, 1.5 = 1.5 and −1.5 = 1.5.
Both 1.5 and −1.5 have the same absolute value of 1.5, because 1.5 is 1.5 units from 0 (to the right) on the number line and −1.5 is 1.5 units from 0 (to the left).
Thus, −n = n for any nonzero number n, and 0 = 0.
Two unequal numbers with the same absolute value are called opposites.
For example, −1.5 is the opposite of 1.5, and 1.5 is the opposite of −1.5.
Note that the only number that is equal to its opposite is 0 (zero).
GMAT Math Review  Arithmetic : Index for Real Numbers
GMAT Math Review  Arithmetic : Practice Exercise for Real Numbers
