A polymer plant with a production capacity of $10,000$ tons per year has an overall yield of $70\%$, on mass basis ($kg$ of product per $kg$ of raw material), The raw material costs Rs. $50,000$ per ton. A process modification is proposed to increase the overall yield to $75\%$ with an investment of Rs. $12.5$ crore. In how many years can the invested amount be recovered with the additional profit? ________________