Question 6
There are 81 soldiers lined up in a row with numbers 1 to 81. In each of the rounds, the soldiers call out 1, 2, 3, 1, 2, 3, 1, 2, 3, .... All soldiers who count 1 and 3 are removed from the line. The process continues until only one soldier is left. What is the number of that last soldier left on the line?