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This dummy needs math help

What's up guys. Smiley face.

Most jewellery is actually a mixture of gold and copper. A jeweller is re-working a few pieces of gold jewellery into a new necklace. Some of the jewellery is 84% gold in mass, and the rest is 75% gold by mass. The jeweller needs 15 g of 80% gold for the necklace. How much of each alloy is used.

Anyway, this isn't homework but I saw this question and it puzzled me.
Thanks in advance and have a great day!