Formula Columns Where My Workers Record The Amounts Of Work They Do During The Day

I have three columns where my workers record the amounts of work they do during the day. The columns are labelled as follows:

Column A – “Correspondences’’
Column B – “Linking’’
Column C – “Allocating”

When a task has been completed an “X’’ is put in the relevant column (which is then summed at the bottom). I use these sums to calculate each workers productivity in the following way – number of “x’’ divided by number of days worked. This is a simple formula, however, each “x” now equates to time – which is causing me problems. An “x’’ is Column A equals 1 hour, Column B 1 hour and Column C 2 hours.

I am looking for someone to help me create a formula which will calculate all possibilities in the above situation above. Please bear in mind that a worker can do one of these tasks during the course of a week, two or all three .