Im look for help in building an expenditure calculator

what i am having problems with is I am using radio buttons to multiply form field values

[radio1] = x 1 (week)
[radio2] = x 4 (month)
[radio3] = x 52 (year)

So if you fill out a field and select [radio2] it multiplies the field by 4

If you then select [radio1] it reverts to the original value

Using JQuery what's the best way to do this.

The issue I am having is I need to know the previous selected radio as well as the new one so I know whether to divide or multiply