$15$ litres of mixture contains $20 \%$ alcohol and the rest water. If $3$ litres of water be mixed with it,the percentage of alcohol in the new mixture would be:

Three containers have volumes in the ratio $3:4:5$. They are full of mixtures of milk and water. The mixtures contain milk and water in the ratio of $4:1$,$3:1$,and $5:2$ respectively. The contents of all these three containers are poured into a fourth container. The ratio of milk and water in the fourth container is

Find the sub-duplicate ratio of $81: 64$.

The ratio of two numbers is $4:5$. If both numbers are increased by $4$,the ratio becomes $5:6$. What is the sum of the two numbers?

Four years ago,the father's age was three times the age of his son. The total of the ages of the father and the son after four years will be $64$ years. What is the father's age at present (in years)?

The ratio of the number of boys and girls in a school of $720$ students is $7: 5$. How many more girls should be admitted to make the ratio $1: 1$?