Simplify the following text for a Grade 6 native English-speaking person.
Original: Varieties of LPG bought and sold include mixes that are primarily propane ( C3H8 ) , primarily butane ( C4H10 ) and , most commonly , mixes including both propane and butane , depending on the season -- in winter more propane , in summer more butane .
Simplified: Very often , mixes of propane and butane are sold .