It’s not really quite so simple. Suppose you weigh 200 pounds with a 20% body fat. If you lost 20 pounds of only fat, then in theory you would weigh 180 at 10% body fat.
However, even if no muscle is lost, with every pound of fat lost, there is also the losing of water weight (water weight you won’t necessarily gain back). So this loss of 20 pounds might only put you down to say a 15% body fat (this only an example, I don’t know what the real ratio of fat to water loss is).
Although I remember reading that a relatively lean person needs to burn 2000 calories to lose one pound, and a pound of fat is 3500 calories (whereas an obese person would need to burn closer to 3000 calories), so for every four pounds of fat, you’ll also drop 3 pounds of water (assuming you’re pretty lean).
I don’t remember my sources, and I can’t seem to find them, so if anyone else knows more about this, feel free to chime in.
I realize you were just discussing definitions, but I thought this would be useful to you as well.