A mixture of milk and water contains 40% milk. From this mixture 15 liter mixture is taken out and replaced with 12 liter water hence new percentage of milk becomes 25%. find the quantity of mixture initially.

Let initial quantity be x liters with 40% milk (0.4x milk, 0.6x water). Removing 15 liters removes 0.4*15=6 liters milk and 0.6*15=9 liters water. Remaining: 0.4x-6 milk, 0.6x-9 water. Adding 12 liters water gives: 0.4x-6 milk, 0.6x-9+12=0.6x+3 water. New total: x-15+12=x-3 liters. New milk percentage: (0.4x-6)/(x-3)=25/100=1/4. Cross-multiply: 4(0.4x-6)=x-3, so 1.6x-24=x-3, thus 0.6x=21, x=35 liters. Option A is correct.