Find the ratio in which rice at ₹ $7.20/kg$ should be mixed with rice at ₹ $5.70/kg$ to produce a mixture worth ₹ $6.30/kg$.

If $5 \ kg$ of potato costing $Rs. 5$ per $kg$ and $3 \ kg$ of potato costing $Rs. 4$ per $kg$ are mixed,find the average cost of the mixture per $kg$?

$A$ sum of ₹ $64$ is made up of $80$ coins which are either $100$ $paise$ or $50$ $paise$ coins. How many coins are of $50$ $paise$?

In what proportion must tea worth $75$ paise per packet be mixed with tea worth ₹$5.50$ per packet so that the mixture may cost ₹$4.50$ per packet?

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?

$A$ mixture of $48$ litre of milk and water contains $10 \%$ of water. How much water must be added to make $20 \%$ water in the new mixture?