Let's find the price per ounce!
6 ounces costs $9, so the price for each ounce is
($9)/(6 oz) = 1.5 $/oz
Now we can find how much 15 ounces will cost by multiplying the cost per ounce by the number ounces we want.
(1.5 $/oz)*(15 oz) = $22.50.
Let me know if that makes sense, and if you have any other questions feel free to shoot me a direct message!