% ?

Okay, so . . .
I'm chewing this horrible tasting gum right now.
Orbit. Spearmint. But! The package promises
that this gum will whiten my teeth
and remove 40% of the stains.
Soon I will only have 60% of my teeth stained!
I keep chewing and thinking about this. Tell me.
Someone.
How do they determine what % of my stains will vanish?
Or rather . . . remain?
Will 40% of my teeth be whiter than the rest
if I just chew on, say, my right side?
I keep picturing this. 4 out of 10 teeth white,
6 out of 10 stained . . .