How to sum datagridview column in last column through checkboxes and also calculate fine

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C#4.0

I have one Datagridview containing six columns

S_Id | Apr | May | Jun | Fine | Total
1001 | 100 | 100 | 100 |      |
1002 | 200 | 200 | 200 |      |
1003 | 300 | 200 | 200 |      |

besides this there are three check boxes also available.
like Chck_Apr, Chck_May, Chck_Jun,

My question is if I select Chck_Apr then the amount of Apr column should display in total columns. And also calculate fine after April 15 with 5 Rs. per day till date of today. and should display in Fine Column lik e

S_Id | Apr | May | Jun | Fine | Total
1001 | 100 | 100 | 100 | 25   | 100    
1002 | 200 | 200 | 200 | 25   | 100
1003 | 300 | 200 | 200 | 25   | 100

note:Fine I have calcualated in range (April 16 to April 20)

like wise If I select Chck_Apr & Chck_Ma both same time then it should display like

S_Id | Apr | May | Jun | Fine | Total
1001 | 100 | 100 | 100 | 155  | 200    note: Date assumed today's (May 16)
1002 | 0   | 200 | 200 |  5   | 200    Note: Fine has calculated after May 15 
1003 | 0   | 200 | 200 |  0   |  0

like wise for next month.

Note for fine schedule:
April Fine after Aprll 15, May Fine Ater May 15 and June Fine After June 15..


Someone please help............

What I have tried:

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