A lot of questions on simple interest are asked in different competitive exams. You are required to have mastery on the concept of Simple Interest in order to solve Compound Interest questions.
Simple interest questions don’t have much variation. Here are the questions you need to find out: Principal, interest, rate, time or amount. Questions on two interest rates for different times may also be asked.
All questions are based on a single basic formulae, but to increase speed, direct formulae are required too.
Amount = Principal + Interest
\[A = P + SI\]
Interest = Amount – Principal
\[SI = A – P\]
Rule  Description 

1  Simple interest = \[\frac{Principal \times Time \times Rate}{100}\] i.e. \[S.I. = \frac{P \times R \times T}{100}\] 
2  Principal (P) = \[\frac{100 \times S.I.}{R \times T}\] 
3  Rate (R) = \[\frac{100 \times S.I.}{T \times P}\] 
4  Time (T) = \[\frac{100 \times S.I.}{P \times R}\] 
5  If rate of Simple Interest differs from year to year, then \[S.I. = P \times \frac{\left(R_1 + R_2 + R_3 + .....\right)}{100}\] 
6  Amount = Principal + Interest
i.e. 
7  If \[\frac{1}{x}\] part of a certain sum P is lent out at R_{1} % SI, \[\frac{1}{y}\] part is lent out at R_{2} % SI and the remaining \[\frac{1}{x}\] part at R_{3} % SI and this way the interest received by 1, then \[P=\frac{1\times100}{\frac{R_1}{x}+\frac{R_2}{y}+\frac{R_3}{z}}\] 
8  If a sum of money becomes n times in T years at simple interest, then formula for calculating rate of interest will be given as \[R=\frac{100\left(n1\right)}{T}\%\] 
9  If a sum of money at a certain rate of interest becomes n times in T_{1} years and m times in T_{2} years, then formula for T_{2} will be given as \[T_2=\frac{\left(m1\right)}{\left(n1\right)}\times T_1\] 
Rule  Description 

1  Simple Interest = \[\frac{Principal \ \times \ Rate \ \times \ Time}{100}\]

2 

3  If there are distinct rates of interest for distinct time periods i.e.
Then, Total S.I. for 3 years = \[\frac{P\left(R_1t_1+R_2t_2+R_3t_3\right)}{100}\] 
4  If a certain sum becomes ‘n’ times of itself in T years on Simple Interest, then the rate percent per annum is.

5  If a certain sum becomes n_{1} times of itself at R_{1}% rate and n_{2} times of itself at R_{2}% rate, then,

6  If Simple Interest (S.I.) becomes ‘n’ times of principal (P) i.e. \[S.I. = P × n\], then

7  If an Amount (A) becomes ‘n’ times of principal (P) i.e. \[A = Pn\], then

8  If the difference between two simple interests is ‘x’ calculated at different annual rates and times, then principal (P) is \[\frac{x\times100}{(difference\ in\ rate)\times(difference\ in\ time)}\] 
9  If a sum with simple interest rate, amounts to ‘A’ in t_{1} years and ‘B’ in same t_{2} years, then

10  If a sum is to be deposited in equal installments, then, Equal installment = \[\frac{A\times200}{T\left[200+\left(T1\right)r\right]}\] where

11  To find the rate of interest under current deposit plan, \[r=\frac{SI\times2400}{n\left(n+1\right)\times\left(deposited \ amount\right)}\] where, n = no. of months. 
12 
If certain sum P amounts to Rs. A_{1} in t_{1} years at rate of R% and the same sum amounts to Rs. A_{2} in t_{2} years at same rate of interest R%. Then,

13  The difference between the S.I. for a certain sum P_{1} deposited for time T_{1} at R_{1} rate of interest and another sum P_{2} deposited for time T_{2} at R_{2} rate of interest is \[SI=\frac{P_2R_2T_2P_1R_1T_1}{100}\] 