Chapters
The simplest definition of interest is that it is the amount of money that is paid in addition to the amount borrowed. The total interest is directly proportional to the amount borrowed and the duration of the loan. This might be a little bit difficult to understand, let's take help from an example for more clarity. For example, Bill wants to borrow 
dollars from the bank for a year and the bank told Bill that they will charge 
interest on the loan. It means that Bill will receive dollars from the bank but he needs to pay interest when returning the loan. The of dollars is 
dollars. That dollars is the "interest"(represented by "I") and is the "interest rate"(represented by "r") but in slang, people still consider as interest. It means that when Bill will return dollars to the bank, after a year, he also needs to pay the interest hence he needs to pay 
dollars to the bank. In that dollars, Bill paid interest + the amount borrowed.
The amount borrowed is called the principal(represented by "P"). In conclusion, Bill paid interest + principal after a year to the bank. Since we talked about special words, there are special words for Bill and the bank as well. We will call Bill as the Borrower and the bank will be Lender. There is another problem, in this whole example, we consider the time period as one year, what if Bill wants to borrow for two years? The question is, will this affect interest? Yes, it will. The bank offered interest rate for a year, which means that if the number of years changes, so will the interest. At that time, Bill would pay dollars after a year but now(since he is borrowing for two years), he needs to pay 
dollars for two years. To calculate interest in an easy way, we have a formula that will calculate interest which is:
(If time is expressed in years)
Where:
| Concept | Name | Symbol |
|---|---|---|
| Amount borrowed | Principal | P |
| Loan duration | Time | t |
| Interest rate | Rate | r |
| Additional amount | Interest | I |
However, the above formula is specified for the interest calculated for years. Every bank has a different interest calculating system, some calculate it in years, while some in months, and you might be a bit surprised to know that some calculate it in days. Does that affect the above formula? Yes, it does but don't worry, if you want to calculate the interest in months then use the below formula:
(If time is expressed in months)
And if you want to calculate the interest in days then:
(If time is expressed in days)
Examples
Calculate the amount of simple interest that is paid over a period of five years on a principal of 
dollars at a simple interest rate of 
.
Since the time is in years:
dollars
Calculate the total amount paid in six months on a principal of 
dollars at a simple interest rate of 
.
Since the time is in months:
dollars
Total amount = 
How long will it take a principal of 
dollars at a simple interest rate of 
to become dollars?
Since the time is in years:
And we need to find time then:
years
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