In what ratio must water be mixed with milk to gain $16 \%$ on selling the mixture at cost price?

Vessels $A$ and $B$ contain mixtures of milk and water in the ratios $4:5$ and $5:1$ respectively. In what ratio should quantities of mixture be taken from $A$ and $B$ to form a mixture in which milk to water is in the ratio $5:4$?

The average weekly salary per head of all employees (supervisors and labourers) is ₹ $100$. The average weekly salary per head of all the supervisors is ₹ $600$,while the average weekly salary per head of all the labourers is ₹ $75$. Find out the number of supervisors in the factory if there are $840$ labourers in it.

In a laboratory,two bottles contain a mixture of acid and water in the ratio $2:5$ in the first bottle and $7:3$ in the second. The ratio in which the contents of these two bottles should be mixed such that the new mixture has acid and water in the ratio $2:3$ is

In a mixture of $60\, L$,the ratio of acid and water is $2:1$. If the ratio of acid and water is to be $1:2$,then the amount of water (in $litres$) to be added to the mixture is:

$A$ person has ₹ $5000$. He invests a part of it at $3 \%$ per annum and the remainder at $8 \%$ per annum simple interest. His total income in $3$ years is ₹ $750$. Find the sum invested at different rates of interest (in ₹).