How do I lock cell value even if other values change

[Guest]

How do I lock cell value even if the number that created the cell value
change? I am trying to make excel calculate a midterm grade. That value
would remain frozen while the overall grade changes.

Example

December 15/05
Subject A = 74
Subject B = 69
Midterm Grade is (74+69)/2 - This value would be locked or frozen

June 23/06
Subject A = 81
Subject B = 77
Midterm Grade is (74+69)/2 - from previous calculation because it was locked
Final Grade (81+77)/2

So far I have tried to use expressions like $G$3

Subject A Subject B Midterm Final
74 69 71.5

but whenever I change the values in Subject A or B, the value in Midterm
changes also.

Please help

 

[Guest]

Uou will need additional columns for the final results. Obviously, when you
change the midterm results, the mudterm average will change accordingly.

Something like
Subject A Subject B Midterm Avg Final Avg
Midterm Final Midterm Final
74 81 69 77 71.5 79

Let's say this starts in Col A, then in E3 enter
=IF(OR(A3="",C3=""),"",(A3+C3)/2
In D3 enter =IF(OR(B3="",D3=""),"",(B3+D3)/2
or something to that effect. It will change if you want to include midterm
average in final average.