Basic - opposite functions on excel

I am attempting to create a monthly budget on excel. 

In one cell (e.g. A1) I have the cost of my accommodation per week. In a separate cell(e.g. B1) I have the cost of that accommodation per month which I have calculated using the formula (=A1*4). Is it possible to have a function in B1 so that I can input how much the accommodation would be per month and have A1 display how much the accommodation would be per week (using =B1/4) so that I could input values into either cell and the other would give me the correct value? So the formula would be opposite? or is this not possible?

Thanks for any help