If $4x - 12y = 20$,then $5x - 15y = \ldots$

Solve the pair of linear equations $2x + y = 40$ and $x + y = 30$. If the solution satisfies the equation $y = mx + 6$,find the value of $m$.

Solve the following pair of equations by the method of cross multiplication:
$ax + by - a + b = 0$
$bx - ay - a - b = 0$

Comparing the equation $y = -3$ with the standard form $ax + by + c = 0$,we get $a = \dots$

$A$ mother tells her son,"Five years ago,$I$ was five times as old as you were. After ten years,$I$ will be two times as old as you will be." Represent this situation as a pair of linear equations in two variables.

Solve the following pairs of linear equations: $\frac{2}{x} + \frac{3}{y} = \frac{9}{xy}, \quad \frac{4}{x} + \frac{9}{y} = \frac{21}{xy} \quad (x \neq 0, y \neq 0)$