Set 14: Systems of Equations

Choice D is correct. Choice D is the correct answer. We use substitution since $y = 2 x$ is already isolated. Step 1: Substitute $y = 2 x$ into $3x + y = 25$: $$$3x + (2 x) = 255x = 25$x = 5 Step 2: Find $y$ using $y = 2 x$: $y = 2(5) = 10$ Solution: $(5, 10)$ Verification: $3(5) + 10 = 15 + 10 = 25$ ✓ Strategic Tip: Substitution works beautifully when one variable is already isolated. Choice A is incorrect because $3(4) + 8 = 12 + 8 = 20 \neq 25$. Choice B is incorrect because $3(6) + 12 = 18 + 12 = 30 \neq 25$. Choice C is incorrect because $3(3) + 6 = 9 + 6 = 15 \neq 25$.