relevent costs

Dear John, the question as shown as follows: material quantity required inventory value/kg realizable value replacement cost/kg W 500kg 0 - - 8 X 500kg 300kg 5 3 7 Y 500kg 400kg 8 9 11 Z 100kg 100kg 10 12 15 material Y is in regular use by the company. Materials X and Z are no longer in regular use. there is no alternative use for Material X but the 100kg of material Z could be used instead of 150 kg of material V on a different job. material V costs $9 / kg and there is currently none of this material in inventory. what is the relevant cost of these material for the job under consideration? the X which I did is that (300*3+200*7), but the answer is that (500*3), I don't know why? many thanks