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?

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?

$A$ mixture contains $80 \%$ acid and the rest water. What part of the mixture should be removed and replaced by the same amount of water to make the ratio of acid to water $4:3$?

$729 \ ml$ of a mixture contains milk and water in the ratio $7:2$. How much more water is to be added to get a new mixture containing milk and water in the ratio $7:3$? (in $ml$)

$A$ drum contains $80$ $litres$ of ethanol. $20$ $litres$ of this liquid is removed and replaced with water. $20$ $litres$ of this mixture is again removed and replaced with water. How much water (in $litres$) is present in this drum now?

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: