Given,

Percentage of copper = 30%

Percentage of nickel = 42%

∴ Percentage of zinc = {100 – (30 + 42)} %

= 28 %

Now,

Mass of zinc in 1 kg of the alloy = (28 × 1/100) kg

= 0.28 kg

= 0.28 × 1000 g

= 280 g