I''m working on my computer as usual, and after I open another excel file from my co-worker, all the formulas freeze. For example, cell A is 1, cell B is 2 and cell C is the total of cell A and B, which is 3. Originally, when I change the number in cell A to 6, the number C will change to 8. Now, I have to double click on cell C and hit enter and then cell C changes to 8. Hello!

This is normal behavior. The calculations won't reflect the correct information until the cell "accepts" the new number. "So won’t you give this man his wings What a shame To have to beg you to see We’re not all the same What a shame" - Shinedown

I guess that depends on your definition of "normal". This would only be "normal" behavior if your Calculation option was set to manual, which I'll wager is extremely rare for most users. While it may be true that you might have to take some action to have the data "accepted", most users do not have to double click on the cell containing the formula and hit enter. Typically most users can simply hit Enter or Tab or an Arrow or click anywhere in the sheet, etc. after typing in the new data. Look at: Tools > Otions > Calculation Tab > Automatic

DerbyDad03: Thanks, I will check the option Hello!