A goldsmith combined an alloy that costs $4.20 per ounce with an alloy that costs $1.70 per ounce. How many ounces of each were used to make a mixture of 200 oz costing $2.25 per ounce?

So your finding an average so you already know the alloy that cost 1.70 that there will be more of that

1.60 150 240

4.20 50 210 this your answer 150 of the 1.60 and 50 of the 4.20 this adds up to be 450/200 which equals 2.25