if the average of three numbers is V. If one of the number is Z and another is Y, what is the remaining number?

V = (y + z + remain.nmbr) / 3
3v = y + z + remaining number

Remaining number = 3v - y - z

Let consider that the remaining number is: X, then

X + Y + Z 
------------- = V  ===> multiply both sides by 3 
      3

X + Y + Z = 3V ===> X = 3V - Y - Z

so, the remaining number is: 3V - Y - Z

hopefully my answer is clearly enough