Question

Tina would like to melt down her existing gold jewelry to make a 20 ounce gold necklace that is 90% pure gold. After melting her jewelry, she is left with two batches. One batch is 75% pure. The other batch is 96% pure. How much of the 96% pure melted gold should Tina mix with the 75% pure melted gold to obtain 20 ounces of 90% pure gold

Answer 1

Say x is the amount of 75% pure gold and y is the amount of 96% gold used to make the 20 oz necklace. This means:

x+y=20

We also know that a specific amount of each batch will be used to make a 90% pure batch. This means:

0.75x+0.96y=0.9

You have two equations and two variables now - can you take it from here?