Percent increase and decrease
Multiply by (1 + rate) to increase and (1 - rate) to decrease.
A worked example
A $80 coat is reduced 25%. New price = 80 x (1 - 0.25) = 80 x 0.75 = $60.
The mistake people actually make
Two discounts do not add. 20% then 25% off is 0.8 x 0.75 = 0.6, so 40% off overall, not 45%.