Answer:
You can't solve this unless you have the amount of the other alloy.
Explanation:
For example, the question should be something like:
A jeweler is using 10 pounds of alloy that is 2% platinum. He has another alloy that is 5% platinum. He wishes to combine these mixtures to form a mixture that is 3% platinum. How many pounds of the second alloy should be added to the first to get a mixture that is 3% platinum?