limiting factor and shadow price

Q plc makes two products – Quone and Qutwo – from the same raw material. The selling price and cost details of these products are as shown below: Quone Qutwo $ $ Selling price 20.00 18.00 Direct material ($2.00 per kg) 6.00 5.00 Direct labour 4.00 3.00 Variable overhead 2.00 1.50 ––––– ––––– 12.00 9.50 ––––– ––––– Contribution per unit 8.00 8.50 The maximum demand for these products is 500 units per week for Quone, and an unlimited number of units per week for Qutwo What is the shadow price of these materials, if material were limited to 2,000 kgs per week? . i didn't get , the shadow price measure of how much they would be willing to pay to gain more of a scarce resource over and above the normal price shouldn't we add one extra KG to 2000 then i make equations and solve to get the quantity of Qutwo the the subtract it from the old quantity?