I've begun a spreadsheet to track donated hours for a project. Some people
have donated a specific number of hours; others have donated a number of
hours multiplied over a 3-month period. I need to add both columns to get a
somewhat clearer count of the total number of hours donated. Is this clear
enough?

Example:

Column A - Name
Column B - Number of Hours Donated
Column C - X (if they've donated x number of hours each week)
Column D - =SUM(B*C)

I need to filter out Column B from D if C=X. It seems so simple, but I
don't know the right formula!