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

Two numbers are in the ratio of $5: 7$. If $25$ is subtracted from each,they are in the ratio of $35: 59$. Find the difference of the two numbers.

The difference between a two-digit number and the number obtained by interchanging the digits is $36$. What is the difference between the sum and the difference of the digits of the number if the ratio between the digits of the number is $1:2$?

If $A: B = 3: 4$ and $B: C = 8: 9$,then find the value of $A: B: C$.

$A$ mixture of milk and water is kept in two separate containers. The ratio of milk to water in one container is $5:1$ and in the other container is $7:2$. In what ratio should the mixtures from these two containers be mixed so that the quantity of milk in the new mixture becomes $80\%$?

If Rs. $872$ is divided into three parts,proportional to $\frac{1}{2}: \frac{2}{3}: \frac{3}{4},$ then the first part is