Finding A missing # From the AVERAGE

Can anyone help me with this? I have about 3000 lines with 3 columns and the average of these 3. I'm missing one set of values that's needed to find the known average. Below is an example of want i'm trying to do.


Column A Column B Column C Column D (avg)
25 X 45 35

the basic rule for this is below but i'm not sure how to input this in excel

25 + X + 45/3 = 35
70 + X = 105 (multiplied both sides by 3 canceling 3 on the left side out)
X = 35 (subtracted both sides by 70)

This again is a basic calculation but I dont know how to use it in excel and I dont wont to have to do this to all 2892 lines manually.

Any help would be great.

Thx

Darryl

Try putting this in cell E2 and drag down:

=D2*3-SUM(A2:C2)

Awesome! thx a bunch.it worked out great..I only tried the formula from PJ but thx for all the help and support!