I hope this helps

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I assumed you the scale is 4.0 (This means your maximum converted numeric number is 4.0 ==> 100.00 = 4.00). This most of the times is given by University regulation.
For instance: you got your math 4.00 which is A and its credit is 3. You must 4*3 then result will be 12. (save this some where, I mean 12).
your algebra is B , refer to your school rules, probably B is equal to 3.33 and numbers between 70 to 75. Now you have your 3.33 , you need to times this to your algebra credit which should be 3. then we have 3.33 * 3 = 9.99.
After you do all your syllabus like this, you have the sum. then you divide the sum of all your marks ,for example in my example: 12+ 9.99=22.99 into the total credit which is 6 in here. 23/6 = 3.8 over 4 which is "A", I think.
all the best
