Question: (a) Let A be a set of seven (distinct) natural numbers none of which exceeds 21. Prove that the sums of the elements in all the nonempty subsets of A are not distinct.
(b) Improve the result of (a) by showing that the result holds under the assumption that the integers of A do not exceed 23.
(c) Assume none of the elements of A exceeds 12. At least how many subsets of A must have the same sum?