How do I calculate the missing number if I have the average

Question

I have the average of 3 numbers but missing 1 of the 3.

E.g. 3+6+9 = 18 and the average of these is 6

but how do i calculate

X in this   2+4+X = 12 and average is 4

I know the answer to the above example :) but need the formular for MS Excel as I have 60 eq of this nature.

Answer 1

2 + 4 + X = 12

6 + x = 12

x = 6

Let's say you have the first number (2 in the example) in cell A1, the second number (4 in the example) in cell A2, and the total (12 in the example) in cell A3.  In cell A4 you can enter the formula =A3-A2-A1

If you don't know the total, then it's like this:

Average = Ave

First number = N1

Second number = N2

The 3 below is because you have 3 numbers (N1, N2, and x.

Ave = (N1 + N2 + x) / 3

3 * Ave = N1 + N2 + x

3 * Ave - N1 - N2 = x

If the first number (N1) is in cell A1, the second number (N2) is in cell A2, and the average (Ave) is in cell A3, then you can put this formula in cell A4:

=3*A3-A1-A2

Answer 2

Add 2+4 

Subtract result from 12

Answer:6