CSIR-NET General Aptitude: Short Notes on Simple Interest!

By Neetesh Tiwari|Updated : July 6th, 2021

Are you looking for some short and reliable notes during your CSIR-NET preparations? Then, you have come to a perfect place!

Candidates preparing for their CSIR NET exam might need to get some short study notes and strategies to apply while preparing for the key exam of their life. At this point, We at BYJU'S Exam Prep come up with short notes on the Simple Interest, which comes under the Quantitative Aptitude section of the General Aptitude syllabus

Our experienced subject-matter experts have meticulously designed this set of short notes on Simple Interest to give you the most standard set of study materials to focus on. In this cut-throat competitive world, students need to prepare themselves with the best study materials to help them learn and for their future. So, here we are offering the best study notes that are reliable and can be used by the students during their preparations for the upcoming CSIR-NET 2021 exam.

Are you looking for some short and reliable notes during your CSIR-NET preparations? Then, you have come to a perfect place!

Candidates preparing for their CSIR NET exam might need to get some short study notes and strategies to apply while preparing for the key exam of their life. At this point, We at BYJU'S Exam Prep come up with short notes on the Simple Interest, which comes under the Quantitative Aptitude section of the General Aptitude syllabus

Our experienced subject-matter experts have meticulously designed this set of short notes on Simple Interest to give you the most standard set of study materials to focus on. In this cut-throat competitive world, students need to prepare themselves with the best study materials to help them learn and for their future. So, here we are offering the best study notes that are reliable and can be used by the students during their preparations for the upcoming CSIR-NET 2021 exam.

Simple Interest

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Example 2: If the principal is 100 Rs. The difference of Simple Interest for 4yrs and 6yrs is Rs 8. Calculate the rate of simple interest.
Solution: In simple interest questions, interest always remains the same for a year if the principal, rate of interest is constant for the same.
Let Interest for 4 yrs is I then interest for 6 yrs is (I+8)
interest for 2 yrs is Rs. 8
interest for 1 yr = 4
rate of interest = (4/100) × 100 = 4%

Example3: If the amount is (10/9) times of Principal and rate of interest and time both are numerically equal. Then, what is the rate of interest per annum?
Solution: Let Principal is P. Given, numerically R = T
Interest = Amount – principal
I = (10/9)P – P
I = P/9 (Interest is in the multiples of Principal)
Now, I =[(P×R×T)/100]
P/9 = (P× R× T)/100
R2 = 100/9 (using, R=T)
R = (10/3)%
We can also say the time period is (10/3)years.


Short approach:
Whenever Interest is in multiple of Principal and Rate of Interest and Time period is equal.

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Annual Instalments for Simple Interest:


Let's discuss a real example to understand  instalment concepts:
A person deposits Rs.140 to the bank every year for up to 5 years. The bank gives him a 5% rate of interest simple annually. And at the end of 5 years, he gets the total amount of Rs.770
So, 140 is the instalment, time is 5 years rate of interest is 5% and the amount of debt is Rs.770
This Instalment is also known as the annual payment. Debt is the total amount, so don’t confuse between these two terms.


Installment =   where A = debt, r = rate of interest and t = time period

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Example4: What annual payment will discharge a debt of Rs.848 in 4yrs at 4% per annum simple interest?

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In case if you forget the formula then how to approach this question.
Let instalment is X. There are 4 instalments and rate of interest is also 4%
Debt (A) = four installments + (r%) × installments × (0+1+2+… (t-1))
So, 848 = 4X + (4%)(X)(0+1+2+3)
      848 = 4X+
      848 = 4X+
      848 = 424X/100
      X = 200

Some Important examples based on Simple Interest.

Example5: A sum amounts to Rs. 702 in 2 years and Rs. 783 in 3 years. Calculate the sum, rate of interest and the amount after 5 years?
Solution: Amount for 2 years(A2) = 702
Amount for 3 years (A3)= 783
Interest for 1 year (I) = 783-702 = 81
So Sum = A2 – 2I = 702 – 2×81
              = 702-162 = 540
rate of interest = (81/540)×100
                             = 15%
Amount after 5 years = Sum+5I
                                    = 540+ 5×81
                                    = 945


Example6: A sum of money doubles itself in 3 yrs at simple interest. In how many yrs will it amount to 8 times itself?
Solution: Doubles in 3 yrs
 3 times in 3× 2 = 6yrs
4 times in 3× 3 = 9yrs
8 times in 3× 7 = 21yrs


Example7: Atul and Vijay are friends. Atul borrowed a sum of Rs.400 at 5% per annum simple interest from Vijay. He returns the amount with interest after 2 yrs. Vijay returns to Atul 2% of the total amount returned. How much did Atul receive?
Solution: After 2 yrs, amount returned to Vijay = 400+ (400*5*2)/100 = Rs 440
Amount returned to Atul = 2% of 440 = 8.8


Example8: Rs.4000 is divided into two parts such that if one part be invested at 3% and the other at 5%, the annual interest from both the investments is Rs. 144. Find each part.
Solution: Let the amount lent at 3% rate be Rs.X, then amount lent at 5% rate is 4000-X
So, 3% of X + 5% of (4000-X) = 144
   5% of 4000 – 2% of X = 144
  200 – 2% of X = 144
  2% of X = 56
  X = (56/2)×100
 X = 2800
And 4000 -X = 1200.


How to solve this Question by Alligation Method:
First, we will calculate the net rate of interest for Rs. 144 on 4000
So, net rate = (144/4000)× 100 = 3.6%


Apply Allegation:

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