A milkman sells the milk at cost price but mixes some water in it hence gets a profit of 40%. Find the quantity of water in a mixture of 42 litres. एक दूधवाला दूध को क्रय मूल्य पर बेचता है लेकिन कुछ पानी मिलाता है अतः 40% का लाभ प्राप्त करता है। 42 लीटर मिश्रण में पानी की मात्रा ज्ञात कीजिए ।

40% profit means selling milk at cost price but the quantity is 140% of actual milk (since profit% = adulteration%). Let milk be m liters and water be w liters. Total volume m+w=42. Since 40% of milk is replaced by water, w=0.4m. Substituting: m+0.4m=42, so 1.4m=42, giving m=30 and w=12 liters. The profit percentage directly equals the percentage of water added.