# How to add interest?

Classical mathematics is represented not only by numerical or alphabetic expressions, but also by percentages.

Percentage is the hundredth part of the number. For example, 1% of 100 = 1; 10% of 100 = 10.

Interest can be added. Let us examine by examples how to add percentages to a number, and also how to add percents to the sum of numbers.

## The percentage addition to the number

- 50 + 5% = ?

We will solve such an example by composing proportions of the form:

- 50 is 100%
- x is 5%
- x is our unknown number.

Solution proportions: x = (50 * 5) / 100%

We get the number x = 2.5.

Thus, 50 + 5% = 50 + 2.5 = 52.5.

- 23 + 11% = ?

The solution can be simplified a bit by avoiding making proportions.

- We write in the numerical form of 11%. Knowing that the percentage is the hundredth part of the number, we get: 11% = 11/100. Hence, 11% = 0.11 of the number we need.
- Find 11% of 23. For this 23 * 0.11 = 2.53
- We calculate the sum of 11% and 23. 23 + 2, 53 = 25.53

As a result, we get the result: 25.53.

## The addition of interest to the amount

Consider a more complex example: 6 + 28 + 50% =?

- In examples of this type,% are considered not from a specific term (in this case, 6 or 28), but from the sum of terms.
- We get 34 + 50% =? This operation is already known from the first two examples, so the amount can be found in any way.
- 50% is half the number. 50% of 34 = 17.
- As a result, the sum is 34 + 17 = 51.

Of course, in addition to adding numbers and percentages, you can also subtract percentages from a number. With this material you can read by reading the article How to deduct interest. This is very useful for understanding, because now you will understand what discounts (or more precisely, the amount of the percentage of discounts) are offered to you in various stores.