Whenever there are variables in the question and numbers in the answer choices, think Plugging In the Answers. In (A), j = 6, and k = -6. Plug these two values into the first equation to get -24 – 8(6) = 12(-6). Solve for both sides of the equation to get -24 – 48 = -72, or -72 = -72. Therefore, the values work for the first equation. Plug the values into the second equation to get 3 + ​$\frac{5}{3}$​(-6) = -​$\frac{7}{6}$​(6). Solve both sides of the equation to get 3 + (-10) = -7, or -7 = -7. Since the values given in (A) work in both equations, the correct answer is (A).

First, you know the new proportion must be less than the current 0.34 for biofuels (because the total amount spent on alternative energy is increasing, but the amount spent on biofuels is remaining the same), so you can eliminate (D). Next, determine the amount that will be spent on biofuels in 2017 by multiplying 0.34 by the total of $254 million: 0.34 × 254 = $86.36 million. Because 57 million new dollars will be spent on alternative energy, the new total will be 254 + 57 = $311 million. Divide $86.36 million by $311 million to get the new proportion: ​$\frac{86.38}{311}$​= 0.28, which is (C).

The formula for compound interest is ​$A=P(1+r)^t$​, where P is the starting principle, r is the rate expressed as a decimal, and t is the number of times the interest is compounded. Melanie received less than 5% interest, so you can eliminate (B) because 1.05 = 1 + 0.05, indicating she was receiving 5% interest. You can also eliminate (C) because over the course of a year the interest is compounded 4 times, not ​$\frac{1}{3}$​of a time. Because Melanie invested $1,100 at what she thought was 5% compounded 4 times (12 months in a year ÷ 3 months per period), she expected 1,100(1 + 0.05)4 = $1,337.06 after a year. Instead, she has 1,337.06 – 50 = $1,287.06 after one year. Because t is in years in the answer choices, make t = 1 in (A) and (D) and eliminate any choice which does not equal 1,287.06. Only (A) works.