1) Choose any 6 numbers from first 7 natural numbers, express the other number using these 6 numbers exactly once and only one operator. This should work for any choice of 6 numbers. What is the operator that has to be chosen?
2) Replace 6 by (n-1) and 7 by n in (1), find for which n, this logic works. If the chosen numbers have to be used at least once, find the other n values for which this logic works.