The three everyday percentage questions — percent of a number, what percent X is of Y, and percentage change — answered instantly.
Subtract the old value from the new value, divide by the old value, multiply by 100. A result of +25 means a 25% increase; −25 means a 25% decrease.
Multiply the price by (1 + percentage/100). For example, $80 plus 20% tax is 80 × 1.2 = $96.