summing in excel using sumifs help required

so, assuming your student's names are in column A and the rest of the subjects are in columns B through H as follows ... English (B1), History (C1), Mathematics (D1), Chemistry (E1), Physics (F1), Computer (G1) and Biology (H1) and the subjects are weighted as you've noted above, maybe this will work for you...
=IF(H2="",SUM(B2:G2)/520,SUM(B2:F2,H2)/530)

EDIT: pointed out I reversed null and not null.

This may be an alternative since I can't tell what relevance you are putting on the values in the parens Mathematics (85) for example, I'm assuming you are weighting them.

=IF(H2="",(SUM(B2,C2,D2*0.85,E2*0.85,F2*0.75,G2*0.75)/520),SUM(B2,C2,D2*0.85,E2*0.85,F2*0.75,H2*0.85)/530)

You need to clarify what you mean by Bio = 0 and Bio ="A" and the same for Comp. If there is an "A" the cell for Bio or Comp cannot hold a value at the same time without being a text cell.
I think a workbook with realistic entries and results is necessary.
Welcome to the forum.

It will make it easier to help you if you include an Excel workbook. (not a picture of a workbook please)

Attach a sample workbook. Make sure there is just enough data to demonstrate your need. Include a BEFORE sheet and an AFTER sheet in the workbook if needed to show the process you're trying to complete or automate. Make sure your desired results are shown, mock them up manually if necessary.

Remember to desensitize the data.

Click on GO ADVANCED and use the paperclip icon to open the upload window.

View Pic

Change the references from relative to absolute. I didn't check the formula for accuracy just the references so that you can fill down without the references changing:

Formula:

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