Thus kg is the ratio component of one part.
Therefore the part for copper is times the single part and the part for zinc is times the single part
In the end be sure to add both part to see if you get the original composition, does it add up to kg?
For the second one, make the division of fraction into multiplication of fraction. To multiply fraction, the numerators are multiplied and the denominators are multiplied. Then do as the other stages (Same steps).
If you find that the values are too high, then you can simply the fraction before proceeding with the arithmetics.