A number is first increased by 10% and then decreased by 10%. Then net percentage increase or decrease in the number is:

1% decrease

10% fraction = 1/10, then

If increase the fraction will be 11/10 and,

If decrease the fraction will be 9/10. 

                 old value      New value 

+ 10%        10                      11

- 10%        10                        9

              ------------------------------------

                  100                     99

Net decrease change = (100 - 99)/100 × 100

⇒ Net decrease change = 1/100 × 100 = - 1%