include column in sum formula if checkbox is checked

I want to set up a "what if" spreadsheet which uses checkboxes.

Lets say that columns B, C, D, & E contain data in rows 5 through 25. Column F is the sum column.

I want to put in checkboxes at row 2 of B, C, D, & E. If checked, column F will sum the data in the column. If unchecked, the column is omitted from the sum.

Will someone be kind enough tell me how to write the formulas? I know how to "insert" a checkbox. But I'm stumped on the format control.

Thanks.

Thanks much!