The total weight of a father and his son is $70 \, kg$. The weight of the son is one-sixth of the weight of his father. Find the weight of the father and the son.

Which of the following is a linear equation in two variables?

There are some students in the two examination halls $A$ and $B$. To make the number of students equal in each hall,$10$ students are sent from $A$ to $B$. But if $20$ students are sent from $B$ to $A$,the number of students in $A$ becomes double the number of students in $B$. Find the number of students in the two halls.

If $I$ add $1$ to each of the two given numbers,then their ratio is $1:2$. If $I$ subtract $5$ from each of the numbers,the ratio is $5:11$. Find the numbers.

The angles of a triangle are $x, y$ and $40^{\circ}$. The difference between the two angles $x$ and $y$ is $30^{\circ}$. Find $x$ and $y$.

The sum and difference of the reciprocals of the ages of a son and father are respectively $\frac{5}{40}$ and $\frac{3}{40}$. Represent this as a pair of linear equations in two variables,where $x$ is the age of the son and $y$ is the age of the father.