Here, $P=$ money deposited per month $=$ ₹ 800 $n=$ Time for which the money is deposited $=1\,\frac{1}{2}\,\text{years}_{\;\text{, }\;}=18\,\text{months.}\,$Let the rate of interest be $r\%$ per annum, then $\therefore \;\; \text{Interest}\;\; = P \times \frac{n(n+1)}{2 \times 12} \times \frac{r}{100}$ $\,=800 \times \frac{18\times 19}{2\times 12} \times \frac{r}{100}$ $\,=\text{₹}\,114\,r$Total money deposited $=18 \times 800 = \text{₹}\,14,400$Since money deposited + interest $=$ Maturity value $14,400+114\,r = 15,084$ $114\,r = 15,084 - 14,400$ $114\,r = 684$ $r\,=\,\frac{684}{114}=6$Hence, rate of interest $=6\%$ p.a.