Three numbers are in the ratio of $3:4:5$. The sum of the largest and the smallest equals the sum of the second and $52$. The smallest number is

The mean proportional between $64$ and $81$ is

The sum of the present ages of a father and his son is $90$ years. $10$ years ago,the ratio of their ages was $5:2$. The present age of the father is (in years):

If $4 \, \text{years}$ ago the ratio between the ages of $P$ and $Q$ was $5:6$ and the sum of their ages at present is $52 \, \text{years}$, what is the ratio of their present ages?

The ratio of the present ages of $R$ and $S$ is $11:17$. $11$ years ago,the ratio of their ages was $11:20$. What is $R$'s present age (in years)?

What will be the duplicate ratio of $2: 7$?