Quote:
If \(\frac{y}{x} ≠ 1\) or 0, what is the value of \(\frac{y}{z}\)?
(1) \(|x + y|=|x + z|\)
(2) \(|x - y|=|x - z|\)
Official Explanation:If y divided by z can’t equal 1, then y and z themselves are not equal. You can use logic to figure this out or you can manipulate
the non-equation by multiplying both sides by z:
\(\frac{y}{z}≠1\)
\(y≠z\)
(1) INSUFFICIENT: Test some cases here. If \(x = 0\), then \(|y| = |z|\). Remember that y and z cannot be the same number! This would
work, then, if \(y = 2\) and \(z = –2\) (or vice versa). In this case, \(\frac{y}{z}= –1\). (As long as x and y are the same number but opposite in sign,
you can choose any values you want, and the quotient will be –1.)
If, on the other hand, \(x = 1\), then \(|1 + y| = |1 + z|\). Solve for the positive version:
\(1 + y = 1 + z\)
\(y = z\)
That is an illegal response, since y can’t equal z. Try the negative version:
\(1 + y = –(1 + z)\)
\(1 + y = –1 – z\)
\(y + z = –2\)
Pick two values that make this statement true. For example, if \(y = –3\) and \(z = 1\), then \(\frac{y}{z} = –3\). There are at least two possible
values for \(\frac{y}{z}\), so this statement is insufficient.
(2) INSUFFICIENT: Test some cases again. If \(x = 0\), then \(|–y| = |–z|\). Remember again that y and z cannot be the same number! This
would work, then, if \(y = 2\) and \(z = –2\) (or vice versa). In this case, \(\frac{y}{z} = –1\).
If, on the other hand, \(x = 1\), then \(|1 – y| = |1 – z|\). Since solving for the negative version worked better last time, start with the
negative version this time:
\(1 – y = –(1 – z)\)
\(1 – y = –1 + z\)
\(2 = y + z\)
Pick two values that make this statement true. For example, if if \(y = 3\) and \(z = –1,\) then \(\frac{y}{z} = –3\). There are at least two possible
values for \(\frac{y}{z}\), so this statement is insufficient.
(1) AND (2) SUFFICIENT: For each statement alone, testing \(x = 0\) produced the same outcome, so at the least, y and z could be
“opposites” (the same number but opposite signs) and \(\frac{y}{z} = –1\). Are there other cases, though, that would work for both
statements?
Take a look at the full versions of the two statements that didn’t produce the illegal outcome \(x = y\); that is, use the negative
version of each:
From statement (1) \(x + y = –(x + z)\) which becomes \(y + z = –2x\)
From statement (2): \(x – y = –(x – z)\) which becomes \(2x = y + z\)
Notice anything? There are similar terms in those equations. Remember that the problem asks about y and z, so manipulate the
first equation to drop the x terms:
(1) \(2x = –y – z\)
(2) \(2x = y + z\)
Set the two right-hand sides equal and simplify:
\(–y – z = y + z\)
\(0 = 2y + 2z\)
\(0 = y + z\)
This final equation proves that y and z have to be opposites: if \(y = 2\), then \(z = –2\); if \(y = 3\), then \(z = –3\); and so on. In any case, then, \(\frac{y}{z} = –1.\)
The correct answer is
.