Irene deposited \$200 in a bank on 1st January and on the first day of each of the following months. At the end of June, when the interest was calculated, he found that his account balance was \$1206.30. Find the rate of a simple interest paid by the bank?
$S_n = \frac{n(A_1+A_n)}{2}\\ S_n = 1206.30\\ A_1=200\\ A_n=\;?\\ n=6$
How to calculate the interest paid by the bank?
$A_n = A_0 + ni$, so $i = \frac{A_6-A_0}{6}$, as we have simple interest here.