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6.Interest
hard
If the ratio of $C.I. \& S.I.$ interest for the same principal and rate of interest for $2$ years is $1.2,$ find the rate of interest (In $\%$)
A
$300$
B
$15$
C
$140$
D
$40$
Solution
Here, $C I=P\left[\left(1+\frac{R}{100}\right)^{2}-1\right]$
$=P\left[\left(1+\frac{R^{2}}{(100)^{2}}+\frac{2 R}{100}\right)-1\right]=\frac{P R}{100}\left[\frac{R}{100}+2\right]$
$\Rightarrow S I=\frac{P \times 2 \times R}{100} \Rightarrow \frac{C I}{S I}=1.2=\frac{\frac{P R}{100}\left[\frac{R+200}{100}\right]}{\frac{2 P R}{100}}$
$\Rightarrow \frac{ CI }{ SI }=\frac{200+ R }{200} \Rightarrow 1.2=\frac{200+ R }{200} \Rightarrow R =40 \%$
Standard 13
Quantitative Aptitude
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