Find the sub-duplicate ratio of $81: 64$.

$15$ litres of mixture contains $20 \%$ alcohol and the rest water. If $3$ litres of water be mixed with it,the percentage of alcohol in the new mixture would be:

Find a fourth proportional to the numbers $60, 48, 30$.

If $3P = 2Q$ and $2Q = 3R$,then what is $P:Q:R$?

Two numbers are in the ratio of $5:6$. If $5$ is subtracted from each number,the ratio becomes $4:5$. The numbers are:

$A$ fraction which bears the same ratio to $\frac{1}{27}$ that $\frac{3}{11}$ does to $\frac{5}{9}$ is equal to