Choice Dhoice C is the correct answer.

1. Equations: $x + y = 20$, $2x + y = 30$
2. Subtract 1st from 2nd: $(2x + y) - (x + y) = 30 - 20 \rightarrow x = 10$
3. Find y: $10 + y = 20 \rightarrow y = 10$

Choice Dhoice D is incorrect because$2(5)+15=25 eq 30$.

Choice Dhoice B is incorrect because$2(8)+12=28 eq 30$.

Choice Dhoice C is incorrect because$2(6)+14=26 eq 30$.