pls help ASAP

A jeweler wants to make a silver alloy to be used to make necklaces. How many ounces of a silver alloy that costs $5.50 per ounce should be mixed with one that costs $8.00 per ounce to make a new 20-ounce alloy that costs $7.00 per ounce?
$5.50 grade oz
$8.00 grade oz

Thank you :)