A recurring deposit credits Rs 400 in the first month and Rs 30 more in each following month.
In which month does the total credited first reach Rs 9000?

$a=400,d=30$, so $S_n=\frac{n}{2}$$[800+30(n-1)]=15n^2+385n$. Then $S_{14}=15(196)+385(14)=8330$, still short of 9000, while $S_{15}=15(225)+385(15)=9150$, which reaches it. So the target is met in the 15th month. Stopping at 14 leaves Rs 670 to go, and 16 and 12 are not the first month to reach the target. Note the answer is a month number, not the amount 9150.