How many liters of water should be added to a $30$ liters mixture of milk and water containing milk and water in the ratio $7:3$ such that the resultant mixture has 40% water in it?

A mixture of alcohol and water contains 35% alcohol by weight. if 25 g water is added to such a mixture of 100 g, then the percentage of alcohol in the mixture is

The ratios of copper to zinc in alloys A and B are 3:4 and 5:9, respectively. If A and B are taken in ratio 2:3 and melted to form new alloy C, then the ratio of copper to zinc in C is: