Exclamation Mathematical comm. question need help!!!

you are given a finite collection of distinct quantities and told thier sum. One of them is chosen(although you are not told which one this is), and you are told the sum of every pair of distinct quantities that contains it. You deduce that the chosen quantity is equal to the difference between this sum of pairs ant the first given pair, divided by two less that the total number of quantities.

Express this result in symbols and hence explain why it is true.

I dont really understand how to go about this question, soo any help would be much appreciated, thanks(Happy)