In a concert, 20 singers will perform. For each singer, there is a (possibly empty) set of other singers such that he wishes to perform later than all the singers from the set.
Can it happen that there are exactly 2010 orders of the singers such that all their wishes are satisfied?
(IMO-2010 Shortlist, Problem C1)
Solution
3. See IMO-2010 Shortlist, Problem C1.
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