Logic and sets · Logic puzzles · Deduction from constraints · Counting argument · River crossing
Ana, Biljana, Vesna and Gordana crossed a river in a canoe in the following way. There were three trips from the left bank to the right bank, and on each of them the canoe carried exactly two of the girls, one of whom did the rowing. On each of the two trips from the right bank back to the left, the canoe carried exactly one girl. Ana is able to row only when she is alone in the canoe, and Biljana only when she is alone or together with Vesna. It is also known that every one of the four girls rowed at least once.
Which of them rowed twice?
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Serbian Regional Competition (Okruzno takmicenje) 1996, high school grade I, problem 5. The 1996 regional paper was not split into categories. Organized by the Mathematical Society of Serbia (DMS). Source