Gold is $19$ times as heavy as water and copper is $9$ times. In what ratio should these metals be mixed so that the mixture may be $15$ times as heavy as water?

In what ratio must water be mixed with milk to gain $16 \frac{2}{3} \%$ on selling the mixture at cost price?

Chocolates at $Rs. 12.00$ per dozen are mixed with chocolates at $Rs. 10.00$ per dozen in the ratio $3:5$. Find the price per dozen of the mixture (in $Rs.$).

Gold is $21$ times as heavy as water and copper is $11$ times as heavy as water. In what ratio should these metals be mixed so that the mixture may be $17$ times as heavy as water?

Three glasses of capacity $2 \, L$,$5 \, L$,and $9 \, L$ contain mixtures of milk and water with milk concentrations of $90 \%$,$80 \%$,and $70 \%$ respectively. The contents of the three glasses are emptied into a large vessel. Find the ratio of milk to water in the resultant mixture.

$A$ container contains $50 \, \text{litres}$ of milk. From this container $5 \, \text{litres}$ of milk was taken out and replaced by water. This process was repeated further two times. How much milk is now contained by the container? (in $\text{litres}$)