The whole cost is fixed (Rs 100), so marginal cost is zero and profit is maximised where $MR = 0$, i.e. where total revenue is greatest. …

Because the firm has only a fixed cost (Rs 100) and no variable cost, its marginal cost is zero. A profit maximiser with zero $MC$ produces where $MR = MC = 0$, i.e. where total revenue is at its maximum.

Total revenue $TR = P \times q$:

$TR$ reaches its maximum of Rs 300. The 6th unit adds nothing to $TR$ ($MR = 300 - 300 = 0$), so the equilibrium output is 6 units at a price of Rs 50 ($q = 5$ at Rs 60 yields the same $TR = 300$).

Short run (TC = Rs 100). Profit $= TR - TC = 300 - 100 = \textbf{Rs 200}$ (supernormal profit).

Long run (TC = Rs 100). A monopoly is protected by barriers to entry, so no new firm can enter to compete the profit away. The Rs 200 profit persists in the long run, with the same equilibrium (6 units at Rs 50).

If the total cost were Rs 1000 instead. …