Updating a value to alter all other values used in a fix sum

Hi All

Apologies for the clunky title. I will try to explain more clearly.

I have a scenario where the total amount I can spend is 100. This is current distributed equally over 4 weeks 25,25,25,25.

I want to change one of the weeks to 40 and then have the rest of the weeks alter in proportion so that the total stays at 100 i.e. (100/40)/3 = 20

I can do this if I know which week I am changing and I have attached this as an example, however I cant see how I would create a formula that allowed me to change any week and have the other three weeks alter.

I have a few more advanced questions but hopefully someone can point me in the right direction to start me off!

Thanks