$A$ sum of $Rs. 6.40$ is made up of $80$ coins which are either $10$-paise or $5$-paise coins. How many $5$-paise coins are there?

Two casks of $48$ $litres$ and $42$ $litres$ are filled with mixtures of wine and water,the proportions in the two casks being respectively $13:7$ and $18:17$. If the contents of the two casks are mixed,and $20$ $litres$ of water is added to the whole,what will be the proportion of wine to water in the resultant mixture?

In an examination out of $480$ students,$85 \%$ of the girls and $70 \%$ of the boys passed. How many boys appeared in the examination if the total pass percentage was $75 \% ?$

$8$ litres of water is added to $32$ litres of a solution containing $20 \%$ of alcohol in water. What is the approximate concentration of alcohol in the solution now? (in $\%$)

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

$A$ 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 $Rs.$)