Interest:- Interest rate is the amount charged, expressed as a percentage of principal, by a lender to a borrower for the use of assets. Interest rates are typically noted on an annual basis.

Types of Interest:- There is two types of Interest

a) Simple Interest 

b) Compound Interest

Terms used in order to find interest:-

Principle:- Principle amount is the original amount of money, the amount before any interest is applied. Principle is asssociated with anything that earns interest (like a savings account) or accrues debt like a car loan, debt that needs to be repaid.(We also mention Sum deposit instead of Principle)

Rate of interest(R%):- It is defined as the proportion of an amount loaned which a lender charges as interest to the borrower, normally expressed as an annual percentage. It is the rate a bank or other lender charges to borrow its money, or the rate a bank pays its savers for keeping money in an account.

Rate of interest is the percentage value of interest per year

Time:-  In order to find Simple interest we always take time in yearly basis. For eg- 6 months is considered as 1/2 year and 3 months is considered as 1/4 year.

Amount:- Amount is reffered as the sum of Principle and interest. We can also say that amount is sum recieved with interest.

Amount = P+I

Difference between simple interest and compound interest:- 

Simple interest is always accrued on principle.

Compound Interest is always accrued on last amount( ist year compound interest accrued on principle, 2nd year compound interest accrued on the amount of ist year).

Note:- In order to find Simple interest as well as Compound interest we will consider Principle as base hence Principle is always 100%.

1. If a certain sum in T years at R% per annum amounts to Rs. A, then the sum will be

2. The annual payment that will discharge a debt of Rs. A due in T years at R% per annum is .Annual payment=



3. If a certain sum is invested in n types of investments in such a manner that equal amount is obtained on each investment where interest rates are R1, R2, R3 ……, R_n, respectively and time periods are T1, T2, T3, ……, T_n, respectively, then the ratio in which the amounts are invested is
 

4. If a certain sum of money becomes n times itself in T years at simple interest, then the rate of interest per annum is

5. If a certain sum of money becomes n times itself at R% per annum simple interest in T years, then
 

6. If a certain sum of money becomes n times itself in T years at a simple interest, then the time T in which it will become m times itself is given by
 

7. Effect of change of P, R and T on simple interest is given by the following formula:
 

8. If a certain sum of money P lent out at SI amounts to A1 in T1 years and to A2 in T2 years, then
 

9. If a certain sum of money P lent out for a certain time T amounts to A1 at R1 % per annum and to A2 at R2 % per annum, then
 

10. If an amount P1 lent at the simple interest rate of R1 % per annum and another amount P2 at the simple interest rate of R2 % per annum, then the rate of interest for the whole sum is