from 2008 to 2009, the sales of a book decreased by 80%.

from 2008 to 2009, the sales of a book decreased by 80%. If the sales in 2010 was the same as in 2008, by what per cent did it increase from 2009 to 2010 ?

Let sale in 2008 = 100
Sale in 2009 = 20
Sale in 2010 = 100
∴ Required increases = [(100 - 20)/20] x100
= (80/20) x 100 = 400%