A food processing company uses $25,000 \:kg$ of corn flour every year. The quantity-discount price of corn flour is provided in the table below:

$$\begin{array}{|l|l|} \hline \text{Quantity (kg)} & \text{Unit price (Rs/kg)} \\ \hline 1-749 & 70 \\ \hline 750-1499 & 65 \\ \hline 1500 \text{ and above} &60 \\ \hline \end{array}$$

The order processing charges are $Rs$. $500$/order. The handling plus carry-over charge on an annual basis is $20\%$ of the purchase price of the corn flour per $kg$. The optimal order quantity (in $kg$) is _____________