The EOQ (Economic Order Quantity) formula is given by:
\[
EOQ = \sqrt{\frac{2DS}{H}}
\]
Where:
- \( D \) is the annual demand (units/year),
- \( S \) is the ordering cost per order,
- \( H \) is the carrying cost per unit per annum.
From the question, the EOQ = 500 units, and the carrying cost \( H \) = Rs. 2 per unit (assumed, since not directly stated but deduced from reverse calculation).
Let’s rearrange the EOQ formula to find the ordering cost \( S \):
\[
S = \frac{EOQ^2 \cdot H}{2D}
\]
However, under optimal EOQ conditions, the annual ordering cost equals the annual carrying cost.
So,
\[
\text{Annual carrying cost} = \text{Annual ordering cost}
\]
Let the annual ordering cost be \( S \). Since it's stated that the purchasing assistant calculated the carrying cost to be Rs. 500,
\[
\text{Annual ordering cost} = Rs. 500
\]