What part of a mixture consisting of 80 litre mixture having 15\% water should be taken out and replaced with water so that the new percentage of water in the mixture may become 25\%?

Let x litres be removed. Initial water = 15% of 80 = 12 litres. After removing x litres and adding water: remaining water = 12 - 0.15x, new water = 12 - 0.15x + x = 12 + 0.85x. Total mixture remains 80 litres. We need (12 + 0.85x)/80 = 0.25. Solving: 12 + 0.85x = 20, so 0.85x = 8, x = 8/0.85 = 800/85 = 160/17. Fraction to remove = (160/17)/80 = 2/17.