how to calculate cumulative interest

Question

Hello.

   i am in need of to calculate following program kindly help me to do.

my project is:

for ex,

     april month deposit - 300 (int 9%)

     interest            -  2.25

     may month deposit     - 300

     existing amount(apr)  - 300

     total                 - 600 (9%)

     interest              - 4.5

sum of int (2.25+4.5) should display in text box. friends i should calculate intrest like this for 12 months
.

    my coding is:

 int cumu = Convert.ToInt32(textBox3.Text.Trim());

            int totdep = Convert.ToInt32(textBox4.Text.Trim());

            decimal tot = cumu + totdep;

            textBox4.Text = Convert.ToString(tot);

            //Decimal subyearly = Convert.ToDecimal(textBox5.Text.Trim());

            decimal cumuint = ((tot * 10) / 100)/12;

            textBox5.Text = Convert.ToString(cumuint);

actually received amount is textbox3. in textbox4 holds total received in that textbox4 calculating  interest. the interest amount will display in textbox5.

my needs are the sum of interest amount should calculate and display in textbox6    

           

how to calculate cumulative interest and the sum of interest should display textbox in c#.net back end is oracle

Thank you in advance..