calculate coding for the following program

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C#

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.

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

i have completed up to calculate interest. i dunno how to calculate cumulative (ie) 2.25 + 4.5
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 cumuint = ((tot * 10) / 100)/12;
           textBox5.Text = Convert.ToString(cumuint);
           int openbal = Convert.ToInt32(textBox2.Text.Trim());
           int totbal = cumu + openbal;
           textBox2.Text = Convert.ToString(totbal);

   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

Old Balance	Current	Total	Per	month		
0	        300	300	9	2.25		
300      	300	600	9	4.5		
600	        300	900	9	6.75		
900	        300	1200	9	9		
1200     	300	1500	9	11.25		
1500	        300	1800	9	13.5		
1800	        300	2100	9	15.75		
2100	        300	2400	9	18		
2400	        300	2700	9	20.25		
2700	        300	3000	9	22.5		
3000     	300	3300	9	24.75		
3300	        300	3600	9	27		
				175.5	total int	I need to calculate this total


how can i develop the code for this calculation.?

[edit]Spurious Inline code removed, Code block added to preserve formatting - OriginalGriff[/edit]

[edit again]Code block added - use the widget above the text box to surround you code: it does all sorts of useful things! - OriginalGriff[/edit again]