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?

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 ₹):

Pencils at $Rs. 4.20$ per dozen are mixed with pencils at $Rs. 5.40$ per dozen in the ratio $3:5$. Find the price per dozen of the mixture (in $Rs.$).

The ratio in which tea costing $Rs. 192$ per $kg$ is to be mixed with tea costing $Rs. 150$ per $kg$ so that the mixed tea,when sold for $Rs. 194.40$ per $kg$,gives a profit of $20\%$,is:

Two vessels contain milk and water in the ratio $3:2$ and $7:3$. Find the ratio in which the contents of the two vessels have to be mixed to get a new mixture in which the ratio of milk and water is $2:1$.

$A$ cylinder which has the capacity of $8 \text{ litres}$ contains a mixture of oxygen and nitrogen. The volume of oxygen in the cylinder is $16 \%$ 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 $9 \%$ of the total volume. Find the amount of mixture released each time (in litres).