Perform simple math operations on filtered column?

Hello,

I am trying to perform operations such as mulitplying the values of a filtered column by 2 but am having a lot of issues with macros. For example, I tried making a seperate column of the result of multiplying the other column by 2 then pasting values back on the original column after a filter but it only pastes the first filtered value. Can someone post some sample code to do something like this? Thanks