She uses 1/2 quart of yellow paint for every 9/2 quarts of blue paint. Then, she uses 1/2 quart of yellow paint for every 1/2 + 9/2 = 10/2 = 5 quart of paint.
We can use the next proportion:

Solving for x:

She uses 1/10 quart of yellow paint for every quart of paint