Choice C is the correct answer. Let $L$ = Lisa's age now and $S$ = sister's age now.

System: $\begin{cases} L = 2S \\ (L - 5) + (S - 5) = 25 \end{cases}$

Step 1: Simplify second equation: $L + S - 10 = 25$$L + S = 35$

Step 2: Substitute $L = 2S$: $2S + S = 35$$3S = 35$$S = \frac{35}{3}$

Five years ago: Lisa was 15 and sister was 5. Sum: $15 + 5 = 20 \ eq 25$.

Sister = 12. Five years ago: $19 + 7 = 26 \ eq 25$.

$2S + S = 35$, so $S = \frac{35}{3}$ which isn't an integer.

$S = 10$ (sister) Five years ago: $(20-5) + (10-5) = 15 + 5 = 20$

💡 Strategic Tip: Age difference remains constant; work backwards from "years ago."

Choice A is incorrect because if Lisa is 15, sister is 7.5 (not integer).

Choice B is incorrect because if Lisa is 18, sister is 9. Five years ago: $13 + 4 = 17 \ eq 20$.

Choice D is incorrect because if Lisa is 24, sister is 12. Five years ago: $19 + 7 = 26 \ eq 20$.