German silver is an alloy composed of nickel, zinc, and copper in a ratio of 3: 4: 13. how many kilograms of each metal are needed to make 4 kg of this alloy?

You need 3 portions of nickel, 4 portions of zinc, and 13 portions of copper, so that is a total of 3+4+13=20 portions
the 20 portions together weigh 4kg, so each portions weighs 4/20=1/5 kg=0.2 kg
so you will need 0.2*3=0.6 kg of nickel, 0.2*4=0.8 kg of zinc, and 0.2*13=2.6 kg of copper.