In this post we will look into the basic concept and solve some questions on Simple and Compound Interest questions for RRB NTPC. This topic needs an aspirant to remember the formulas related to simple interest and compound interest. One of the most important skill needed to solve problems of Simple and Compound Interest questions for RRB NTPC is simplification technique and placing the given data appropriately.

**Formulas of Simple and Compound Interest:**

Let us look at the formulas of Simple Interest and Compound interest:

Simple interest = (Principal * Rate of Interest * Time period)/100

Compound interest = Principal * (1 + (Rate of interest/100))^{Time period }– Principal

Amount = Principal * (1 + (Rate of interest/100))^{Time period}

In Compound Interest questions, students are generally confused between the amount and the interest earned. You have to subtract the principle from the amount to get the compound interest.

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**Simple and Compound Interest Questions for RRB NTPC:**

Here are some questions which will help you to understand the concept and how to approach these questions in less time.

Q1. A sum of money triples itself in 8 years at a certain rate of SI. How long will it take for it to become 5 times at the same rate of interest?

A) 14 years

B) 15 years

C) 16 years

D) 18 years

Answer key: C

Solution:

Let principle be P

Let rate of interest be R%

Sum of money triple in 8 years = 3P

Simple interest = 3P – P = 2P

2P = (P*R*8)/100

R = 25%

Sum becomes 5 times means it become 5P

Simple interest = 5P – P = 4P

Let Time by which it becomes 5P be T

4P = (P*25*T)/100

T = 16 years

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Q2. At what percentage simple interest per annum a certain sum will double in 10 years?

A) 7%

B) 8%

C) 9%

D) 10%

Answer key: D

Solution:

Let sum be P and rate of interest be R%

2P – P = (P*R*10)/100

R = 10%

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Q3. An amount of 10,000 lent at compound interest at the rate of 6% per year. What would be the compound interest after 2 years?

A) 1200

B) 1221

C) 1236

D) 1250

Answer key: C

Solution:

Principle= Rs 10000

Rate of interest = 6%

Time = 2 years

Compound interest= Principal * (1 + (Rate of interest/100))^{Time period}

= 10000(1 + (6/100))^{2} – 10000

= 1236

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