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6.Interest
medium
The simple and compound interests on a sum of money for $2$ years are ₹ $8400$ and ₹ $8652$ respectively. The rate of interest per annum is (In $\%$)
A
$6$
B
$7.5$
C
$9$
D
$4.5$
Solution
(a) Quicker Method:
For 2 years
$\because$ simple interest $=\frac{200 \times \text { Rate }}{\text { Rate }(\text { Rate }+200)} \times$ CI
$\Rightarrow 8400=\frac{200 r}{r(r+200)} \times 8652$
$\Rightarrow 8400\left(r^{2}+200 r\right)=8652 \times 200 r$
$\Rightarrow 8400 r^{2}=(8652-8400) \times 200 r$
$\Rightarrow 42 r=252$
$\therefore r=6 \%$
Standard 13
Quantitative Aptitude