The percentage increase/decrease from old value (Vold) to new value (Vnew) is equal to the old and new values difference divided by the old value times 100%:
percentage increase/decrease = (Vnew - Vold) / Vold × 100%
Price percentage increase from old value of $1000 to new value of $1200 is caluclated by:
percentage increase = ($1200 - $1000) / $1000 × 100%
= 0.2 × 100% = 20%
Price percentage decrease from old value of $1000 to new value of $800 is caluclated by:
percentage decrease = ($800 - $1000) / $1000 × 100%
= -0.2 × 100% = -20%
The difference d is equal to the initial value V0 times the percentage increase/decrease p divided by 100:
d = V0 × p / 100
The final value V1 is equal to the initial value V0 plus the difference d:
V1 = V0 + d