The .mp3 player finally sells for $99.00 .
Original price . . . $100 .
Increase the original price by 10% .
(multiply by (100% + 10%) = 110% = 1.10)
(1.10) x ($100) = $110 .
Now decrease the $110 by 10% .
(multiply by (100% - 10%) = 90% = 0.90)
(0.90) x ($110) = $99 .