A container holds 200 litres of a solution of acid and water, having 30% acid by volume. Atul replaces 20% of this solution with water, then replaces 10% of the resulting solution with acid, and finally replaces 15% of the solution thus obtained, with water. The percentage of acid by volume in the final solution obtained after these three replacements, is nearest to

200 L with (30%) acid $$\Rightarrow$$ acid = $$0.30\times200=60 L$$

Replace 20% with water: remaining acid $$=60\times(1-0.20)=60\times0.8=48 L$$.

Replace 10% with pure acid: After removing 10% of the mixture, the acid becomes $$48\times0.9=43.2 L$$, then adding back 20 L pure acid $$\Rightarrow acid =43.2+20=63.2 L$$.

Replace 15 with water: acid left $$= 63.2\times(1-0.15)=63.2\times0.85=53.72 L$$.

Final concentration $$= \dfrac{53.72}{200}=0.2686\approx26.86%$$.

The answer is 27%