Answer: Lou needs to add 1/6 gallon of white paint.
Step-by-step explanation:
To make a gallon of a paint color, Lou mixes 1/6 gallon of yellow paint with 2/3 gallon of blue paint. It means that the amount if yellow paint and blue paint that Lou mixed is
1/6 + 2/3 = 5/6 gallon
Lou fills the rest of the can with white paint to make one gallon of paint. It means that the amount of white paint that Lou needs to add to make a full gallon of paint is
1 - 5/6 = 1/6 gallon
