Question

$\large\underbrace{\bf{ \red{Question}}}$

A man invested Rs. 20000 at 10% per annum at simple interest, and another amount at 5% per annum at simple interest. At the end of the year he got 7% interest on the entire investment. Find out his total investment.​

Answer 1

$\large\underline{\sf{Solution-}}$

Given that,

• A man invested Rs. 20000 at 10% per annum at simple interest.

• Another amount at 5% per annum at simple interest.

• At the end of the year he got 7% interest on the entire investment.

Let assume that amount invested at the rate of 5 % per annum be Rs x.

Case :- 1

Principal, P = Rs 20000

Rate of interest, r = 10 % per annum

Time, n = 1 year

We know,

Simple interest (SI) received on a certain sum of money of Rs P invested at the rate of r % per annum for n years is given by

$\boxed{ \rm{ \:SI \: = \: \dfrac{P \times r \times n}{100} \: }}$

So, on substituting the values, we get

$\rm \: SI_1 = \dfrac{20000 \times 10 \times 1}{100}$

$\rm\implies \:\boxed{ \rm{ \:SI_1 = Rs \: 2000 \: \: }} - - - (1)$

Case :- 2

Principal, P = Rs x

Rate of interest, r = 5 % per annum

Time, n = 1 year

So,

$\rm \: SI_2 = \dfrac{x \times 5 \times 1}{100}$

$\rm\implies \:\boxed{ \rm{ \:SI_2 \: = \: Rs \: \frac{x}{20} \: \: }} - - - (2)$

Case :- 3

Principal, P = Rs (20000 + x)

Rate of interest, r = 7 % per annum

Time, n = 1 year

So,

$\rm \: SI_3 = \dfrac{(20000 + x) \times 7 \times 1}{100}$

$\rm\implies \:\boxed{ \rm{ \:SI_3 \: = \: Rs \: \dfrac{140000 + 7x}{100} \: }} - - - (3)$

Now, According to statement,

$\rm \: SI_3 = SI_1 + SI_2$

$\rm \: \dfrac{140000 + 7x}{100} = 2000 + \dfrac{x}{20}$

$\rm \: 140000 + 7x = 200000 + 5x$

$\rm \: 7x - 5x = 200000 - 140000$

$\rm \: 2x = 60000$

$\bf\implies \:x = 30000$

So, his Total investment = 20000 + 30000 = Rs 50000

$\rule{190pt}{2pt}$

Additional Information :-

$\begin{gathered}\: \: \: \: \: \: \begin{gathered}\begin{gathered} \footnotesize{\boxed{ \begin{array}{cc} \small\underline{\frak{\pmb{ \red{More \: Formulae}}}} \\ \\ \bigstar \: \bf{P = \dfrac{SI \times 100}{r \times n} }\\ \\ \bigstar \: \bf{r = \dfrac{SI \times 100}{P \times n} }\\ \\ \bigstar \: \bf{n = \dfrac{SI \times 100}{P \times r} }\\ \\ \bigstar \: \bf{Amount = P\bigg(\dfrac{100 + rn}{100} \bigg) }\: \end{array} }}\end{gathered}\end{gathered}\end{gathered}$