You are going to mix a gallon bucket of window cleaner. the instructions direct you to mix 1 part cleaner to 3 parts water. how much cleaning solution will you need to use? *

The correct ratio is 1 part cleaner to 3 parts water.

Thus:  

    Such a solution would have the combined volume 4 parts.

    Here we want 1 gallon of cleaning solution, so use 1/4 gallon cleaner and 
    3/4 gallon water.  Note that 1/4 gallon + 3/4 gallon = 1 gallon (as desired),
    and that the ratio of cleaning solution volume to water volume is

     1/4 cup
    = 1:3, as required.
      3/4 cup