Given the mean (average) of three numbers and the value of one of the three numbers, is it possible to determine the values of the other two numbers?
For example, given the average of numbers x, y, and 21 is 23, is it possible to find the values of x and y? And could this be applied to where, if you the average of a group of numbers and the values of all but two of the numbers, could you determine the remaining values? (Example: if the average of n numbers is a, knowing n-2 of the values and knowing a, can you determine the other two values?)
Taking the information you have given:${x+y+21\over 3}=23$ , thus $x+y=48$
but there are infinitely many solutions to this equation so no unique solutions. In the general case you have given, you end up with the sum of the 2 numbers being some value so you also end up with infinitely many solutions.