In what ratio must $25 \%$ hydrochloric acid be mixed with $60 \%$ hydrochloric acid to get a mixture of $40 \%$ hydrochloric acid?

$A$ tea seller mixes three qualities of tea $T_{1}, T_{2}$,and $T_{3}$ priced at $Rs. 74$ per kg,$Rs. 68$ per kg,and $Rs. 63$ per kg in the ratio of $1:2:4$. He added some more quantity of $T_{1}$ to $4$ kg of this mixture. He sold this new mixture for $Rs. 84$ per kg,thereby making a profit of $20\%$. How much of $T_{1}$ did he mix with the mixture? (in kg)

Zinc and copper are in the ratio $5:3$ in $400 \, g$ of an alloy. How much copper (in $g$) should be added to make the ratio $5:4$?

$A$ 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 $25\%$?

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.

$20$ litres of a mixture contains $20 \%$ alcohol and the rest is water. If $4$ litres of water are added to it,what will be the percentage of alcohol in the new mixture?