Choice D is the correct answer. We use substitution since $y = 2x$ is already isolated.

Step 1: Substitute $y = 2x$ into $3x + y = 25$: $3x + (2x) = 25$$5x = 25$$x = 5$

Step 2: Find $y$ using $y = 2x$: $y = 2(5) = 10$

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$3(4) + 8 = 12 + 8 = 20 \ eq 25$.

Choice B is incorrect because$3(6) + 12 = 18 + 12 = 30 \ eq 25$.

Choice C is incorrect because$3(3) + 6 = 9 + 6 = 15 \ eq 25$.