Still looking for an answer to a problem

ITEM %VALUE TRIP TRIP TOTAL
Feb Mar
Slab (10%) 40% 100%
Framing (20%) 40%
Electric (15%) 10%

Ok here's my problem, I need to have the Total change on every trip by
multipling the %Value column by the percentage completed of that Item on each
trip.
So for trip in Feb there is only one total which is in the Slab row and it
should be 4%.
The next trip in Mar the total for each Item should be as follows.
Slab 100%
Framing 8%
Electric 1%
On each trip that follows the total needs to update every time I put a new
percentage in the Trip column.

So is there a way to do it.
--
Rodney