Rs 8000 invested at compound interest gives Rs 1261 as interest after

1.	Rs 8000 invested at compound interest gives Rs 1261 as interest after 3 years. The rate of interest per annum is (a) 25% (b) 17.5% (c) 10% (d) 5%
Answer» (d) 5% P = Rs 8000, C.I. = Rs 1261 \(\Rightarrow\) Amount = Rs 9261, n = 3, r = ? \(\therefore\) 9261 = 8000\(\big(1+\frac{r}{100}\big)^3\) \(\Rightarrow\) \(\big(1+\frac{r}{100}\big)^3\) = \(\frac{9261}{8000}\) = \(\big(\frac{21}{20}\big)^3\) \(\Rightarrow\) 1 + \(\frac{r}{100}\) = \(\frac{21}{20}\) \(\Rightarrow\) \(\frac{r}{100}\) = \(\frac{21}{20}\) - 1 =\(\frac{1}{20}\) \(\Rightarrow\) r = \(\frac{100}{20}\)% = 5% p.a.

Rs 8000 invested at compound interest gives Rs 1261 as interest after 3 years. The rate of interest per annum is (a) 25% (b) 17.5% (c) 10% (d) 5%

Answer»

(d) 5%

P = Rs 8000, C.I. = Rs 1261

\(\Rightarrow\) Amount = Rs 9261, n = 3, r = ?

\(\therefore\) 9261 = 8000\(\big(1+\frac{r}{100}\big)^3\)

\(\Rightarrow\) \(\big(1+\frac{r}{100}\big)^3\) = \(\frac{9261}{8000}\) = \(\big(\frac{21}{20}\big)^3\)

\(\Rightarrow\) 1 + \(\frac{r}{100}\) = \(\frac{21}{20}\)

\(\Rightarrow\) \(\frac{r}{100}\) = \(\frac{21}{20}\) - 1 =\(\frac{1}{20}\)

\(\Rightarrow\) r = \(\frac{100}{20}\)% = 5% p.a.