I have an inventory list where different people check out certain products.
I need to be able to have a formula that adds the value of a certain product
for a certain salesman. This is an example of what I have.

Date Product Amount salesperson
10-Oct Pencil 5 Jacob
15-Oct Pen 10 Brian
16-Oct Paper 2 Brian
20-Oct Pencil 11 Jacob
25-Oct Pencil 7 Brian

and i want it to show that for this month Jacob sold a total of 16 pencils.