A total of $60$ vouchers were given on the day. The number of ₹ $1,000$ vouchers added to $3$ times the number of ₹ $500$ vouchers, gives $100$. Express the given information as a system of linear equations in two variables. Hence, find the number of vouchers of each type by matrix method. OR (b) Given that $P = \begin{bmatrix} 2 & -1 \\ 3 & 4 \end{bmatrix}$, $Q = \begin{bmatrix} 5 & 2 \\ 7 & 4 \end{bmatrix}$ and $R = \begin{bmatrix} 2 & 5 \\ 3 & 8 \end{bmatrix}$, find a matrix $S$ such that $PQ - RS$ is a null matrix.