How long does it take for a sum of Rs 2500 to become Rs 12500 at 10 simple interest 1 point

7.	An automobile financier claims to be lending money at simple interest, but he includes the interest every six months for calculating the principal. If he is charging an interest of 10%, the effective rate of interest becomes:
Answer: Option B Explanation: Let the sum be Rs. 100. Then, S.I. for first 6 months = Rs. 100 x 10 x 1 = Rs. 5 100 x 2 S.I. for last 6 months = Rs. 105 x 10 x 1 = Rs. 5.25 100 x 2 So, amount at the end of 1 year = Rs. (100 + 5 + 5.25) = Rs. 110.25 Effective rate = (110.25 - 100) = 10.25%

8.	A lent Rs. 5000 to B for 2 years and Rs. 3000 to C for 4 years on simple interest at the same rate of interest and received Rs. 2200 in all from both of them as interest. The rate of interest per annum is:
Answer: Option D Explanation: Let the rate be R% p.a. Then, 5000 x R x 2 + 3000 x R x 4 = 2200. 100 100 100R + 120R = 2200 R = 2200 = 10. 220 Rate = 10%.

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The principle of compounding growth is used extensively in the financial world to transform small savings into a big corpus over time. It’s also the underlying idea behind MBA topics such as time value of money and discounted cash flow (DCF) valuation.

Learn about simple and compound interest concepts as you’ll need them not only for entrance exams but in the real world too, especially after you become rich and famous.
 

Here is a list of some basic definition and formulas to solve problems on Interest.

Principal: This is the sum of money lent or borrowed.

Interest: This is the extra money paid for taking the money as loan. This is often expressed as a percentage.

Say, the interest is 10% on a loan of Rs. 100. Then the interest in amount is Rs. 10 and at the end of the year, the amount to be paid is Rs. 110.

Time: This is the time period for which the money is lent or the time period in which the money has to be returned with interest.
 

Simple Interest

As the name implies, the calculation of simple interest is pretty simple. Multiply the principal amount with the number of years and the rate of interest.

Simple Interest Formula:
Simple Interest = Principal * Time * Rate of interest / 100

Abbreviated as SI = PTR/100
 

Compound Interest

In compound interest, the principal amount with interest after the first unit of time becomes the principal for the next unit.

Say, when compounded annually for 2 years, the principal amount with interest accrued at the end of first year becomes the principal for the second year.

Compound Interest Formula:
Amount = Principal * [1 + Rate of Interest/100]Time period

Abbreviated as Amount = P * [1 + R/100]t, when compounded annually.

Sometimes, the interest is also calculated half-yearly or quarterly.

When compounded semi-annually or half-yearly,

Amount = P[1 + (R/2)/100]2t

When compounded quarterly,

Amount = P[1 + (R/4)/100]4t

Present worth of Principal P due t years hence is given by:

P/[1+ R/100]t

Sample problems and solutions

Let us work on some examples to understand the concepts and the differences.

  Problem 1. A sum of Rs. 25000 becomes Rs. 27250 at the end of 3 years when calculated at simple interest. Find the rate of interest.

Solution:

Simple interest = 27250 – 25000 = 2250

Time = 3 years.

SI = PTR / 100 → R = SI * 100 / PT

R = 2250 * 100 / 25000 * 3 → R = 3%.

  Problem 2. Find the present worth of Rs. 78000 due in 4 years at 5% interest per year.

Solution:

Amount with interest after 4 years = Rs. 78000

Therefore, simple interest = 78000 – Principal.

Let the principal amount be p.

78000 – p = p*4*5/100 → p=13000

Principal = 78000 – 13000 = Rs. 65000

  Problem 3. A certain principal amounts to Rs. 15000 in 2.5 years and to Rs. 16500 in 4 years at the same rate of interest. Find the rate of interest.

Solution:

Amount becomes 15000 in 2.5 years and 16500 in 4 years.

Simple interest for (4-2.5) years = 16500 – 15000

Therefore, SI for 1.5 years = Rs. 1500.

SI for 2.5 years = 1500/1.5 * 2.5 = 2500

Principal amount = 15000 – 2500 = Rs. 12500.

Rate of Interest = 2500 * 100 / 12500 * 2.5 → R = 8%.

  Problem 4. Find the compound interest on Rs. 3000 at 5% for 2 years, compounded annually.

Solution:

Amount with CI = 3000 (1+ 5/100)2 = Rs. 3307.5

Therefore, CI = 3307.5 – 3000 = Rs. 307.5

  Problem 5. Find the compound interest on Rs. 10000 at 12% rate of interest for 1 year, compounded half-yearly.

Solution:

Amount with CI = 10000 [1+ (12/2 * 100)]2 = Rs. 11236

Therefore, CI = 11236 – 10000 = Rs. 1236

  Problem 6. The difference between SI and CI compounded annually on a certain sum of money for 2 years at 8% per annum is Rs. 12.80. Find the principal.

Solution:

Let the principal amount be x.

SI = x * 2 * 8 / 100 = 4x/25

CI = x[1+ 8/100]2 – x → 104x/625

Therefore, 104x/625 – 4x/25 = 12.80

Solving which gives x, Principal = Rs. 2000.

  Problem 7. Find the simple interest on Rs. 5000 at a certain rate if the compound interest on the same amount for 2 years is Rs. 253.125.

Solution:

Let the rate of interest be r.

5000[1+ r/100]2 = 5000+253.125

→ [1+r/100]2 = 5253.125/5000

Solving which gives

[1+ r/100]2 = 1681/1600

→ 1+r/100 = 41/40

→ r = 2.5

Therefore, SI = 5000* 2 * 2.5/ 100 = Rs. 250.

  Problem 8. A certain amount becomes Rs. 5760 in 2 years and Rs. 6912 in 3 years. What is the principal amount and the rate of interest?

Solution:

Interest on Rs. 5760 for 1 year = 6912 – 5760 = Rs. 1152

Therefore, Rate of interest for 1 year = 100*1152/5760*1 = 20%

Let the principal be p.

Then, Principal = p[1+ 20/100]2 = 5760

Solving which gives Principal = Rs. 4000

  Problem 9. How long will it take a certain amount to increase by 30% at the rate of 15% simple interest?

Solution:

Let the principal be Rs. x

Simple interest = x*30/100 = 3x/10

T = 100*SI/PR = 100*3x/10 / x*15 = 2%

Alternatively, this can be solved by considering principal amount to be Rs. 100. Then simple interest becomes Rs. 30.

Then, T = 100*30/100*15 = 2%
 

Simple and Compound Interest Quiz: Solve these problems

Problem 1: Click here

Answer 1: Click here

Problem 2: Click here

Answer 2: Click here

How long will it take for a sum of money to grow from Rs 2500 to Rs 10000 if it is invested at 12.5% pa simple?

At what rate of simple interest can a sum of Rs 12500 amount to Rs 15500 in 4 years?

What will be the compound interest earned on an amount of Rs 12500 in 2 years at 6 Pa?

How long will it take for a sum of money to grow from Rs 1250 to Rs 10000 if it is invested at 12.5% per annum simple interest?