$A$ mixture of $40$ $litres$ of milk and water contains $10 \%$ water. How much water must be added to make $20 \%$ water in the new mixture? (in $Liters$)

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

$300\, gm$ of salt solution has $40 \%$ salt in it. How much salt should be added to make it $50 \%$ in the solution? (in $gm$)

Tea worth ₹ $126/kg$ and ₹ $135/kg$ are mixed with a third variety in the ratio $1:1:2$. If the mixture is worth ₹ $153/kg$,the price of the third variety per kg will be (in ₹):

How much water should be added to $14$ litres of milk worth ₹ $5.40$ a litre so that the value of the mixture may be ₹ $4.20$ a litre? (in litres)

How many $Kg$ of tea,worth ₹ $25$ per $Kg$,must be blended with $30 \ Kg$ of tea worth ₹ $30$ per $Kg$,so that by selling the blended variety at ₹ $30$ per $Kg$ there should be a gain of $10 \%$?