An alloy contains only zinc and copper and they are in the ratio of 7 : 9. If the weight of the alloy is 8 kg, then find the weight of copper in the alloy.
4\(\tfrac{1}{2}\) kg
Total parts = 7 + 9 = 16. Copper = (9/16)×8 = 4.5 kg = 4\(\tfrac{1}{2}\) kg.