Weighted average of available results

Hi everyone. I am relatively new to excel and enjoy using it to monitor my grades throughout my university degree.
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I am trying to have cell F1 show my rolling average taking into consideration the different weightings of the results. Currently, it is a simple C2*E2+C3*E3.... therefore there are 0s in the sum. Column D and E are manually filled out as the semester progresses and more results are made available. Is there a way I can exclude the cells that currently don't have a mark and see the weighted average?