A) 7 yr
B) 10 yr
C) 15 yr
D) 20 yr
Correct Answer: C
Solution :
[c] Let principal be Rs. x and the rate be R% per annum. Then. \[x\times {{\left( 1+\frac{R}{100} \right)}^{5}}=2x\] \[\Rightarrow \] \[{{\left( 1+\frac{R}{100} \right)}^{5}}=2\] (i) Let \[x\times {{\left( 1+\frac{R}{100} \right)}^{t}}=8x\] \[\Rightarrow \] \[{{\left( 1+\frac{R}{100} \right)}^{t}}\] \[=8={{2}^{3}}={{\left\{ {{\left( 1+\frac{R}{100} \right)}^{5}} \right\}}^{3}}\] \[\Rightarrow \] \[{{\left( 1+\frac{R}{100} \right)}^{t}}={{\left( 1+\frac{R}{100} \right)}^{15}}\] \[\Rightarrow \] \[t=15\,\,yr\] |
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