Choice C is the correct answer. Since $y = 3x$ is already isolated, we can directly substitute it into the second equation.

Step 1: Substitute $y = 3x$ into $x + y = 16$: $x + (3x) = 16$$4x = 16$$x = 4$

Step 2: Find $y$ using $y = 3x$: $y = 3(4) = 12$

Solution: $(4, 12)$

Verification: $4 + 12 = 16$ ✓ and $12 = 3(4)$ ✓

💡 Strategic Tip: When one equation is already solved for a variable, substitution is the fastest method.

Choice A is incorrect because$5 + 11 = 16$ ✓, but $11 \ eq 3(5) = 15$.

Choice B is incorrect because$3 + 9 = 12 \ eq 16$.

Choice D is incorrect because$6 + 10 = 16$ ✓, but $10 \ eq 3(6) = 18$.