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6.Interest
hard
In how many years will a sum of ₹ $800$ at $10 \%$ per annum compound interest, compounded semiannually becomes ₹ $926.10$?
A
$1.5$
B
$1.0$
C
$2.5$
D
$2.0$
Solution
(a) Rate of interest $=10 \%$ per annum. So, rate of interest for half-yearly $=5 \%$ Therefore, $A=P \frac{(1+R)^{T}}{100}$
$926.10=800 \frac{(1+5)^{T}}{100}$
$926.10=800 \frac{(100+5)^{T}}{100}$
$926.10=800 \frac{(21)^{T}}{20}$
$\frac{926.1 \times 10}{8000 \times 10}=\left(\frac{21}{20}\right)^{T}$
$\frac{9261}{8000}=\left(\frac{21}{20}\right)^{T}$
$\left(\frac{21}{20}\right)^{3}=\left(\frac{21}{20}\right)^{T}$
Hence, time $=3$ half-years
$=1 \frac{1}{2}$ years
Standard 13
Quantitative Aptitude