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

$49 \ kg$ of blended tea contains Assam and Darjeeling tea in the ratio $5:2$. The quantity of Darjeeling tea to be added to the mixture to make the ratio of Assam to Darjeeling tea $2:1$ is..........$kg$.

$A$ tub contains a mixture of two liquids $P$ and $Q$ in the ratio of $4:1$. When $10 \, L$ of the mixture is taken out and $10 \, L$ of liquid $Q$ is poured into the tub,the ratio becomes $2:3$. How many litres of liquid $P$ were contained in the tub initially?

In what proportion must sugar of $Rs. 17$ per $kg$ be mixed with sugar of $Rs. 29$ per $kg$ to make a mixture of $Rs. 20$ per $kg$?

$A$ 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?

$A$ shopkeeper bought $20 \ kg$ of onions at $Rs. 6.50$ per $kg$ and $30 \ kg$ of onions at $Rs. 7$ per $kg$. He sold the mixture at a total profit of $Rs. 60$. At what price per $kg$ did he sell the mixture of onions? (in $Rs.$)