Calculating a percentage involves finding a proportion of a number in relation to the whole, often expressed as a fraction of 100. Here are a few common methods to calculate percentages:

## 1. **Percentage of a Number:**

To find a certain percentage of a number, multiply the number by the percentage and divide by 100.

**Example:** What is 20% of 150?

`(20/100) * 150 = 0.2 * 150 = 30`

So, 20% of 150 is 30.

## 2. **Percentage Increase or Decrease:**

To calculate the percentage increase or decrease between two numbers, use the following formula:

**Percentage Increase:**

`((New Value - Old Value) / Old Value) * 100`

**Percentage Decrease:**

`((Old Value - New Value) / Old Value) * 100`

## 3. **Percentage of a Whole:**

To find what percentage one number is of another, divide the part by the whole and multiply by 100.

**Example:** What percentage is 40 of 200?

`(40 / 200) * 100 = 0.2 * 100 = 20%`

## 4. **Percentage Points:**

When discussing changes in percentages, it’s important to understand the concept of percentage points. A percentage point is the arithmetic difference between two percentages. For example, an increase from 20% to 30% is a 10 percentage point increase.

## 5. **Calculating Discounts or Markups:**

To calculate the final price after a discount or markup, you can use the following formulas:

**For Discount:**

`Final Price = Original Price - (Original Price × Discount Percentage / 100)`

**For Markup:**

`Final Price = Original Price + (Original Price × Markup Percentage / 100)`

Understanding these basic methods will allow you to calculate percentages in various situations. Excel and most calculators also have built-in functions to simplify these calculations.