Video solution here (3:26):
Key Habit for Speed on Data Sufficiency: We often don't need to actually solve.
[b]We just need to know that we CAN answer the question.
This can save a lot of time on problems with harder calculations.
With 2 UNIQUE LINEAR equations and 2 unknowns (or 1 equation & 1 unknown, 3 equations and 3 unknowns, etc.), we can solve.
Caveats: ⚠️Watch out when it's asking for a "Combo" of two variables. For example, if the question asks for the ratio "X/Y", we can solve with the one equation 2x = 3y. We get "two birds with one stone."
⚠️The equations must be UNIQUE. A common trap is where one equation is just a multiple of the other. For example, x + 2y = 3 and 3x + 6y = 9 are really just the same equation!
⚠️Quadratics often have 2 different solutions for 1 equation. For example, \(x^2 = 9\) has 2 different solutions: x=3 OR x=-3.
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