An alloy contains silver, aluminium and copper in ratio 5 ∶ 4 ∶ 7, the quantity of copper in kg that must be added to 320 kg of mixture, to have the alloy that contain silver, aluminium and copper in the ratio 5 ∶ 4 ∶ 8?
1
10 kg
2
15 kg
3
20 kg
4
50 kg
5
30 kg