I am trying to run a sumproduct formula and can't seem to get it right.

I have daily cash activity listed in column A. Each day has several listings and different categories in column B. I have the amounts in column C.

I want to run a calculation for one month's worth of days (column a) across one category (column b) using the amounts selected in columns A and B from column C.

I thought the sumproduct formula would work, but I am stuck.

Any ideas?