I am asked to find

how many ounces of a solution that is 90% alcohol need to be mixed with 5 ounces of a solution that is 50% alcohol in order to obtain a solution that is 80% alcohol?

I set it up this way, but don't think i did it right, can someone please help me???

.90x+.50(5)=.80y

but then how do I find the solution?