Solve the following pairs of equations:
$\frac{x}{3} + \frac{y}{4} = 4$
$\frac{5x}{6} - \frac{y}{8} = 4$

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 $y$.

Solve the following pair of linear equations in two variables using a graph: $4x + y = 7$ and $16x + 4y = 28$.

The pair of equations $ax - y = 5$ and $4x - 2y = 10$ has an infinite number of solutions. Then $a = \ldots$

Given below is a graph showing two lines. Which of the following statements is true about the solution$(s)$ of the pair of equations represented by these lines?

The two-digit number whose digit at the unit's place is $x$ and the digit at the ten's place is $y$ is three times the sum of its digits. This statement is written as $\ldots \ldots \ldots \ldots .$