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

The milk and water in two vessels $A$ and $B$ are in the ratio $4:3$ and $2:3$ respectively. In what ratio should the liquids in both the vessels be mixed to obtain a new mixture in vessel $C$ containing half milk and half water?

$A$ dishonest milkman professes to sell his milk at cost price but he mixes it with water and thereby gains $25 \%$. The percentage of water in the mixture is?

Tea worth ₹ $126/kg$ and ₹ $135/kg$ are mixed with a third variety in the ratio $1:1:2$. If the mixture is worth ₹ $153/kg$,the price of the third variety per kg will be (in ₹):

In what ratio must water be added to spirit to gain $10 \%$ by selling it at the cost price?

$A$ $12 \text{ litres}$ cylinder contains a mixture of oxygen and nitrogen,the volume of oxygen being $40 \%$ of the total volume. $A$ few litres of the mixture is released and an equal amount of nitrogen is added. Then,the same amount of the mixture as before is released and replaced by nitrogen for the $second$ time. As a result,the oxygen content becomes $10 \%$ of the total volume. How many $litres$ of the mixture is released each time?