Solve the following pair of linear equations by the method of substitution: $2x + 3y = 7$ and $6x + 9y = 23$.

Obtain the solution of the following pair of equations by elimination method:
$\frac{11}{y} - \frac{7}{x} = 1, \, \frac{9}{y} - \frac{4}{x} = 6$; $x \neq 0, \, y \neq 0$

Solve the following pair of linear equations using a graph: $x+y=7$,$5x+2y=20$.

The graph of the pair of equations $3x + 4y = 10$ and $3x + 4y = 15$ represents:

Do the following equations represent a pair of coincident lines? Justify your answer.
$\frac{x}{2} + y + \frac{2}{5} = 0$
$4x + 8y + \frac{5}{16} = 0$

The cost of $4$ pens and $4$ pencil boxes is $Rs.\, 100$. Three times the cost of a pen is $Rs.\, 15$ more than the cost of a pencil box. Form the pair of linear equations for the above situation. Find the cost of a pen and a pencil box (in $Rs.$).