How many litres of water should be added to 30 litres of solution containing concentrated Hydrochloric acid and water in the ratio of 7:3. Such that the resultant mixture has 40\% water in it?

Initial solution: 30L with acid:water = 7:3. Acid = (7/10) × 30 = 21L, Water = (7/10) × 30 = 9L. Let x litres of water be added. New water = 9 + x, Total mixture = 30 + x. Given: (9+x)/(30+x) = 40/100 = 2/5. Cross-multiplying: 5(9+x) = 2(30+x) → 45 + 5x = 60 + 2x → 3x = 15 → x = 5 litres. Option A would give 35% water. Option C would give approximately 36.7% water. Option D would give 43.3% water.