These two basic math formulas are typically taught in, or at least reviewed in, seventh (7th) grade math these days.
Use these simple math formulas for a variety of everyday situations, like figuring out tips on a restaurant meal or other service, other markups, increases or decreases in productivity, clearance and sales price discounts in percentages, etc., etc.
For calculating percentage decreases (practical examples: percentages off a product or service, decreases in productivity by percentage)
1) Subtract the original amount (the original price, the original number of people, the original amount of productivity, etc.) from the new amount (new - original).
2) Take the answer to step 1, and divide that number by the original amount (new - original / original).
3) The final number you come up with (after completing the first two steps): multiply this number by 100 for your final percentage (new - original / original x 100).
For calculating totals after a known percentage increase is factored in (practical examples: tip on a product or service, increases in productivity, other markup increases)
1) Take the original amount (the original price, the original number of people, the original amount of productivity, etc.), and multiply this number by the known percentage increase.
2) Take the answer to step 1, and add that number to the original amount to get your final answer.
Example: A meal cost $25.50 at a restaurant. To figure out the final cost after a 20% tip is factored in, we take the $25.50 and multiply it by 20% (25.50 x 0.2). By doing this, we figure out that the tip itself is going to be $5.10. We then simply add the tip to the original cost of the meal, and we discover that the final cost will be $30.60.