Calculation of Interest 
The interest calculation is an extension of the percentage calculation. Interest is a percent value of one underlying or principal. 
Base of the percentage calculation is the formula: 

Z = Interest
K = Principal
P = Interest rate

The formula can be changed for the value you are looking for 
Interest rate:

Interest:

Principal:


Calculate interest income 
This example calculates the interest earned on investing 3000$ for one year at a fixed rate of 3%. 
Given is the interest rate P = 3 and the capital = 3000. 
We are looking for the interest income Z. 
The interest income is calculated according to the formula:$ 

Calculate interest rate 
This example calculates the interest rate, which is required to receive $ 150 interest in one year, from a capital of $ 3,000 
The capital K = 300 and the interest income Z = 150 are known. 
We are looking for the interest rate P. 
Calculated according to the formula: 


Calculate starting capital 
What amount must be invested in order to receive an interest income of 200 $ at a rate of 5%? This question should be solved in this task. 
The interest rate of P = 5% and the interest income Z = 200 $ are known 
We are looking for starting capital K. 
It is calculated according to the formula: $


Calculate interest income daily 
For example, suppose you want to invest $ 5,000 for 2 months at an annual interest rate of 5%. For this, the interest must be calculated on a daily basis. The formula for calculating the interest income is extended accordingly by the number of days = t. For each month, 30 days, so 360 days for 1 year are assume. 
The advanced formula is:

The capital K = 5000, the interest rate P = 5 and the number of days t = 60 are known 
We are looking for interest income Z. 
This is calculated: $

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