How much tea at $Rs. 4$ a $kg$ should be added to $15 \, kg$ of tea at $Rs. 10$ per $kg$ so that the mixture is worth $Rs. 6.50$ a $kg$? (in $kg$)

There are $65$ students in a class. $39$ rupees are distributed among them so that each boy gets $80$ Paise and each girl gets $30$ Paise. Find the number of boys and girls in that class.

$A$ person has a chemical costing $Rs. 25$ per $litre$. In what ratio should water be mixed in the chemical so that by selling the mixture at $Rs. 20$ per $litre$,he may get a profit of $25\%$?

$A$ mixture of a certain quantity of milk with $16$ litres of water is worth $90$ $P$ per litre. If pure milk is worth $Rs. 1.08$ per litre,how much milk is there in the mixture? (in Litres)

Nikita bought $30 \text{ kg}$ of wheat at the rate of $Rs. 9.50$ per $\text{kg}$ and $40 \text{ kg}$ of wheat at the rate of $Rs. 8.50$ per $\text{kg}$ and mixed them. She sold the mixture at the rate of $Rs. 8.90$ per $\text{kg}$. Her total profit or loss in the transaction was:

$A$ milk vendor has $2$ cans of milk. The first contains $25\%$ water and the rest milk. The second contains $50\%$ water. How much milk should he mix from each of the containers so as to get $12$ litres of milk such that the ratio of water to milk is $3:5$?