If $\frac{5}{x}-\frac{4}{y}=4$ and $\frac{4}{x}-\frac{5}{y}=5,$ then $\frac{1}{x}-\frac{1}{y}=\ldots \ldots \ldots \ldots .(x, y \neq 0)$

The graph of the equation $\ldots \ldots \ldots \ldots$ is parallel to the $Y$-axis.

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 $\ldots \ldots \ldots \ldots .$

Solve the following pairs of equations by the method of elimination:
$5x - 3y = 1$
$2x + 5y = 19$

Find the value of $p$ if the lines represented by the equations $3x - y - 5 = 0$ and $6x - 2y - p = 0$ are parallel.

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.