For a pair of equations $13x - 7y = 19$ and $7x - 13y = 31$,find the value of $x + y$.

The standard form of $2x = 5y$ is............

The angles of a cyclic quadrilateral $ABCD$ are $\angle A = (6x + 10)^{\circ}$,$\angle B = (5x)^{\circ}$,$\angle C = (x + y)^{\circ}$,and $\angle D = (3y - 10)^{\circ}$. Find $x$ and $y$,and hence the values of the four angles.

Four years ago,the sum of the ages of four persons was $40$ years. After two years,the sum of the ages of the same four persons will be ............ years.

The graph of the equation $2x - y = 1$ does not pass through which of the following points?

Solve the following pair of equations by the method of cross multiplication:
$x + y - (a + b) = 0$
$ax - by - (a^2 - b^2) = 0$