I think I'm just lazy to try to memorize another formula. There are just way too many formulas in this world, so why not just try to solve by deriving things (or just use common sense)?
20% increase, then 5% decrease, is simply:
(1.2y)*(.95) = 1.14y (or 14% increase). What's so difficult with that?
or if you do: 7% increase, then 30% decrease, and then 15% increase:
(1.07)*(0.70)*(1.15) ~ 0.86y (or 14% decrease)
kudos if you like me (or my post)