Answer: £30.20
We can say, 4kg of copper is mixed with 1kg of Zinc to make 5 kg of brass.
So to make 7 kg of brass we need to find out how much kg is copper and zinc mixed. Just increase one kg in both the weight, 5kg of copper and 2kg of zinc makes it 7kg of brass. (Technical thinking)
Now that we know the proportions of the metals so find out the costs of each kg of it.
Copper: 1 kg costs £5
5kg costs- 5*5 --> £25
Zince: 1 kg costs £3.20
2kg costs- 3.20 * 2 --> £5.20
Now add them, £5.20 + £25 --> £30.20
So, the cost of 7kg of brass is £30.20
