Keep a cell as a Running Total by changing another cell

I m tired seeking help,

i have a format like this

A1 =days B1= week C1=Month
i want that if A1=6 then b1=6 c1=6 after that if i change a1 value as 4 then B1=10 D1=10 and its keep running B1 till one week and C1 till month

hope you understand

Thanks
Prakash Joshi