31

Part of 1990 IMO Longlists

Problems(1)

Find the number of good subsets of S - ILL 1990 IND4

Source:

S = \{1, 2, \ldots, 1990\}

31

-element subset of

S

is called "good" if the sum of its elements is divisible by

5

. Find the number of good subsets of

S.

symmetrycombinatorics solvedcombinatorics