In a large bowl, combine all-purpose flour, whole-wheat flour, dry milk, baking powder and salt. Mix well.
Stir in oil until evenly mixed.
Store in the refrigerator in a closed container.
To measure Master Mix for recipes, spoon it lightly into a measuring cup and level off with the flat edge of a knife or spatula.
For best quality, use all the mix within a month.
You can substitute margarine or butter for the oil. Cut 1 cup plus 2 Tablespoons margarine or butter into small pieces and add it to the bowl of mixed dry ingredients. Use a fork or pastry blender to mash the margarine or butter into flour until the mixture is crumbly.