costing material problem

Q1. the annual demand for an item of stock is 45 units. the item costs rs 200 per unit to purchase. holding costs are 15% of the unit costs per annum and the ordering costs are rs. 300 per order. the supplier offers a 2% discount for orders in excess of 60 units and a discount of 5% for orders in excess of 90 units. what is the economic lot size?