Question 340389
The average of three numbers is found by adding the three numbers and dividing the sum by three. Let'a call the remaining number x, so we can express the average
{{{(Z+Y+x)/3 = V}}} Solve for x.  Multiply both sides by 3.
{{{Z+Y+x = 3V}}} Now subtract Z and Y from both sides.
{{{x = 3V-Z-Y}}} 
The remaining number is {{{highlight(3V-Z-Y)}}}