costing problem

can anyone tell me the solution of this Question...

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?

thanks