Since there is 320 grams of copper, than 16 multiplied by 20 will be 320. So then you have to multiply 3 times 20 which is 60. So....
3:16 multiplied by 20 will be 60:320.
ANSWER:
60 parts zinc. Hope this helped. Have a great day! A brainliest and (or) thank you is always appreciated. Goodbye!