However, the rule 2 variables & 2 equations is NOT written in stone. It depends on case-per-case basis. Sometimes you can have 2 equations and still not be able to solve them.
We can't use the n variables and n equations tactic when:
--equations aren't linear
--equations aren't distinct (the so-called "evil-twin" situation)
--we have integers-->this restricts the kinds of values variables can take such that you may have sufficiency even if you have fewer equations than variables
In general, the risk in using the tactic is concluding INSUFFICIENCY. So be careful before you conclude a statement is insufficient when using this tactic. But when concluding SUFFICIENCY, there isn't as much risk.
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The question stem provides us with 1 equation and 2 unknowns. Knowing nothing about the properties of the unknowns, we would need an additional equation.
But (1) tells us x is a positive integer. But, as Pradeep showed, y can be a non-integer. It can also be a negative number:
5(4) + 3(-1) = 17
Thus, (1) is insufficient. Eliminate A and D.
(2) provides us with an additional equation. Thus, we have two equations, and two unknowns, and, without solving, we can safely and quickly conclude that (2) is sufficient by itself.
Choose B.
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